diff --git a/src/evaluator/index.test.ts b/src/evaluator/index.test.ts
index f09cf7e..827bf50 100644
--- a/src/evaluator/index.test.ts
+++ b/src/evaluator/index.test.ts
@@ -142,4 +142,27 @@ describe("evaluate()", () => {
       expect(evaluated).toBe(expected);
     });
   });
+
+  describe("function expressions", () => {
+    const cases = [
+      {
+        name: "simple function expression",
+        input: "함수 () { 1 }"
+      },
+    ];
+
+    it.each(cases)("evaluate $name", ({ input }) => {
+      const lexer = new Lexer(input);
+      const parser = new Parser(lexer);
+      const program = parser.parseProgram();
+      const evaluator = new Evaluator();
+      const environment = new Environment();
+      const evaluated = evaluator.evaluate(program, environment);
+
+      expect(evaluated).not.toBeUndefined();
+      expect(evaluated).toHaveProperty("parameters");
+      expect(evaluated).toHaveProperty("body");
+      expect(evaluated).toHaveProperty("environment");
+    });
+  });
 });
diff --git a/src/evaluator/index.ts b/src/evaluator/index.ts
index 2dfb450..5533125 100644
--- a/src/evaluator/index.ts
+++ b/src/evaluator/index.ts
@@ -148,6 +148,11 @@ export default class Evaluator {
 
       throw new Error(`bad prefix expression: prefix: '${node.prefix}' with type: '${typeof subExpression}'`);
     }
+    if (node.type === "function expression") {
+      const parameters = node.parameter;
+      const body = node.body;
+      return { parameters, body, environment: env };
+    }
     if (node.type === "assignment") {
       const varValue = this.evaluate(node.right, env);
 
diff --git a/src/lexer/index.test.ts b/src/lexer/index.test.ts
index 900ac75..301fbaf 100644
--- a/src/lexer/index.test.ts
+++ b/src/lexer/index.test.ts
@@ -8,6 +8,7 @@ import {
   groupDelimiter,
   blockDelimiter,
   keyword,
+  separator,
   illegal,
   end,
 } from "./token";
@@ -21,6 +22,7 @@ import type {
   GroupDelimiter,
   BlockDelimiter,
   Keyword,
+  Separator,
   Illegal,
   End,
 } from "./token";
@@ -122,11 +124,20 @@ describe("getToken()", () => {
       const cases: { input: string, expected: Keyword }[] = [
         { input: "만약", expected: keyword("만약") },
         { input: "아니면", expected: keyword("아니면") },
+        { input: "함수", expected: keyword("함수") },
       ];
 
       it.each(cases)("get group delimiter token '$input'", testLexing);
     });
 
+    describe("separator", () => {
+      const cases: { input: string, expected: Separator }[] = [
+        { input: ",", expected: separator(",") },
+      ];
+
+      it.each(cases)("get separator token '$input'", testLexing);
+    });
+
     describe("illegal", () => {
       const cases: { input: string, expected: Illegal }[] = [
         { input: "$", expected: illegal("$") },
@@ -194,6 +205,22 @@ describe("getToken()", () => {
           end,
         ]
       },
+      {
+        input: "함수(사과, 바나나) { 사과 + 바나나 }",
+        expectedTokens:[
+          keyword("함수"),
+          groupDelimiter("("),
+          identifier("사과"),
+          separator(","),
+          identifier("바나나"),
+          groupDelimiter(")"),
+          blockDelimiter("{"),
+          identifier("사과"),
+          operator("+"),
+          identifier("바나나"),
+          blockDelimiter("}"),
+        ],
+      },
     ];
 
     it.each(cases)("get tokens from input '$input'", ({ input, expectedTokens }) => {
diff --git a/src/lexer/index.ts b/src/lexer/index.ts
index 44544a6..ce1e10b 100644
--- a/src/lexer/index.ts
+++ b/src/lexer/index.ts
@@ -37,6 +37,12 @@ export default class Lexer {
           return Token.blockDelimiter(delimiter);
         }
 
+      case ",":
+        {
+          const separator = this.charBuffer.pop() as typeof char;
+          return Token.separator(separator);
+        }
+
       case "!":
         {
           this.charBuffer.pop();
@@ -95,7 +101,7 @@ export default class Lexer {
             return Token.booleanLiteral(read);
           }
 
-          if (read === "만약" || read === "아니면") {
+          if (read === "만약" || read === "아니면" || read === "함수") {
             return Token.keyword(read);
           }
 
diff --git a/src/lexer/token/index.test.ts b/src/lexer/token/index.test.ts
index fb193ae..12f7f83 100644
--- a/src/lexer/token/index.test.ts
+++ b/src/lexer/token/index.test.ts
@@ -5,6 +5,7 @@ import {
   booleanLiteral,
   stringLiteral,
   groupDelimiter,
+  separator,
   keyword,
 } from "./";
 import type {
@@ -14,6 +15,7 @@ import type {
   BooleanLiteral,
   StringLiteral,
   GroupDelimiter,
+  Separator,
   Keyword,
 } from "./";
 
@@ -108,10 +110,23 @@ describe("group delimiter", () => {
   });
 });
 
+describe("separator", () => {
+  const cases: { input: Separator["value"], expected: Separator }[] = [
+    { input: ",", expected: separator(",") },
+  ];
+
+  it.each(cases)("make separator token for '$input'", ({ input, expected }) => {
+    const token = separator(input);
+
+    expect(token).toEqual(expected);
+  });
+});
+
 describe("keywords", () => {
   const cases: { input: Keyword["value"], expected: Keyword }[] = [
     { input: "만약", expected: keyword("만약") },
     { input: "아니면", expected: keyword("아니면") },
+    { input: "함수", expected: keyword("함수") },
   ];
 
   it.each(cases)("make keyword token for '$input'", ({ input, expected }) => {
diff --git a/src/lexer/token/index.ts b/src/lexer/token/index.ts
index 9ed9277..d7a1b6b 100644
--- a/src/lexer/token/index.ts
+++ b/src/lexer/token/index.ts
@@ -6,6 +6,7 @@ export type TokenType =
   StringLiteral |
   GroupDelimiter |
   BlockDelimiter |
+  Separator |
   Keyword |
   Illegal |
   End;
@@ -19,8 +20,10 @@ type LogicalOperatorValue = "!" | "!=" | "==" | ">" | "<" | ">=" | "<=";
 type BooleanLiteralValue = "참" | "거짓";
 type GroupDelimiterValue = "(" | ")";
 type BlockDelimiterValue = "{" | "}";
-type KeywordValue = BranchKeywordValue;
+type SeparatorValue = ",";
+type KeywordValue = BranchKeywordValue | FunctionKeywordValue;
 type BranchKeywordValue = "만약" | "아니면";
+type FunctionKeywordValue = "함수";
 type EndValue = typeof END_VALUE;
 
 export interface Operator {
@@ -58,6 +61,11 @@ export interface BlockDelimiter {
   value: BlockDelimiterValue;
 }
 
+export interface Separator {
+  type: "separator",
+  value: SeparatorValue;
+}
+
 export interface Keyword {
   type: "keyword";
   value: KeywordValue;
@@ -108,6 +116,11 @@ export const blockDelimiter = (value: BlockDelimiter["value"]): BlockDelimiter =
   value,
 });
 
+export const separator = (value: Separator["value"]): Separator => ({
+  type: "separator",
+  value,
+});
+
 export const keyword = (value: Keyword["value"]): Keyword => ({
   type: "keyword",
   value,
diff --git a/src/parser/index.test.ts b/src/parser/index.test.ts
index 127aa30..8fcb8df 100644
--- a/src/parser/index.test.ts
+++ b/src/parser/index.test.ts
@@ -749,6 +749,76 @@ describe("parseProgram()", () => {
     it.each(cases)("parse $name", testParsing);
   });
 
+  describe("functions", () => {
+    const cases: { name: string, input: string, expected: Program }[] = [
+      {
+        name: "function expression with parameters",
+        input: "함수 (사과, 바나나) { 사과 + 바나나 }",
+        expected: {
+          type: "program",
+          statements: [
+            {
+              type: "expression statement",
+              expression: {
+                type: "function expression",
+                parameter: [
+                  { type: "identifier", value: "사과" },
+                  { type: "identifier", value: "바나나" },
+                ],
+                body: {
+                  type: "block",
+                  statements: [
+                    {
+                      type: "expression statement",
+                      expression: {
+                        type: "infix expression",
+                        infix: "+",
+                        left: { type: "identifier", value: "사과" },
+                        right: { type: "identifier", value: "바나나" },
+                      },
+                    },
+                  ],
+                },
+              },
+            },
+          ],
+        },
+      },
+      {
+        name: "function expression with no parameters",
+        input: "함수 () { 사과 + 바나나 }",
+        expected: {
+          type: "program",
+          statements: [
+            {
+              type: "expression statement",
+              expression: {
+                type: "function expression",
+                parameter: [],
+                body: {
+                  type: "block",
+                  statements: [
+                    {
+                      type: "expression statement",
+                      expression: {
+                        type: "infix expression",
+                        infix: "+",
+                        left: { type: "identifier", value: "사과" },
+                        right: { type: "identifier", value: "바나나" },
+                      },
+                    },
+                  ],
+                },
+              },
+            },
+          ],
+        },
+      },
+    ];
+
+    it.each(cases)("parse $name", testParsing);
+  });
+
   describe("branch statements", () => {
     const cases: { name: string, input: string, expected: Program }[] = [
       {
diff --git a/src/parser/index.ts b/src/parser/index.ts
index 1664714..511a4fc 100644
--- a/src/parser/index.ts
+++ b/src/parser/index.ts
@@ -10,6 +10,7 @@ import {
   makeExpressionStatement,
   makePrefixExpression,
   makeInfixExpression,
+  makeFunctionExpression,
 } from "./syntax-tree";
 import type {
   Program,
@@ -194,6 +195,13 @@ export default class Parser {
       const expression = makePrefixExpression(prefix, subExpression);
       return expression;
     }
+    if (token.type === "keyword" && token.value === "함수") {
+      const parameters = this.parseParameters();
+      const body = this.parseBlock();
+
+      const functionExpression = makeFunctionExpression(body, parameters);
+      return functionExpression;
+    }
     if (token.type === "group delimiter" && token.value === "(") {
       const groupedExpression = this.parseExpression(bindingPower.lowest);
 
@@ -209,6 +217,60 @@ export default class Parser {
     throw new Error(`bad token type ${token.type} (${token.value}) for prefix expression`);
   }
 
+  private parseParameters(): Identifier[] {
+    const parameters: Identifier[] = [];
+
+    const maybeGroupStart = this.buffer.read();
+    if (maybeGroupStart.type !== "group delimiter" || maybeGroupStart.value !== "(") {
+      throw new Error(`expected ( but received ${maybeGroupStart.type}`);
+    }
+    this.buffer.next();
+
+    // early return empty parameters if end of parameter list
+    const maybeIdentifierOrGroupEnd = this.buffer.read();
+    this.buffer.next();
+    if (maybeIdentifierOrGroupEnd.type === "group delimiter" && maybeIdentifierOrGroupEnd.value === ")") {
+      return [];
+    }
+    const maybeIdentifier = maybeIdentifierOrGroupEnd;
+
+    // read first parameter
+    if (maybeIdentifier.type !== "identifier") {
+      throw new Error(`expected identifier but received ${maybeIdentifier}`);
+    }
+    const identifier = maybeIdentifier;
+    parameters.push(identifier);
+
+    // read the rest parameters
+    while (true) {
+      const maybeCommaOrGroupEnd = this.buffer.read();
+      this.buffer.next();
+
+      // break if end of parameter list
+      if (maybeCommaOrGroupEnd.type === "group delimiter" && maybeCommaOrGroupEnd.value === ")") {
+        break;
+      }
+      const maybeComma = maybeCommaOrGroupEnd;
+
+      // read comma
+      if (maybeComma.type !== "separator") {
+        throw new Error(`expected comma but received ${maybeComma}`);
+      }
+
+      // read next identifier
+      const maybeIdentifier = this.buffer.read();
+      this.buffer.next();
+      if (maybeIdentifier.type !== "identifier") {
+        throw new Error(`expected identifier but received ${maybeIdentifier}`);
+      }
+      const identifier = maybeIdentifier;
+
+      parameters.push(identifier);
+    }
+
+    return parameters;
+  }
+
   private parseInfixExpression(left: Expression): Expression | null {
     let token = this.buffer.read();
     if (token.type !== "operator") {
diff --git a/src/parser/syntax-tree/expression/index.ts b/src/parser/syntax-tree/expression/index.ts
index 7bd5d01..e73c6a3 100644
--- a/src/parser/syntax-tree/expression/index.ts
+++ b/src/parser/syntax-tree/expression/index.ts
@@ -1,3 +1,5 @@
+import { Block } from "../group";
+
 export type Expression =
   Identifier |
   NumberNode |
@@ -5,6 +7,7 @@ export type Expression =
   StringNode |
   PrefixExpression |
   InfixExpression |
+  FunctionExpression |
   Assignment;
 
 export interface Identifier {
@@ -40,6 +43,12 @@ export interface InfixExpression {
   right: Expression;
 }
 
+export interface FunctionExpression {
+  type: "function expression";
+  parameter: Identifier[],
+  body: Block;
+}
+
 export interface Assignment {
   type: "assignment";
   left: Identifier;
@@ -79,6 +88,12 @@ export const makeInfixExpression = (infix: InfixExpression["infix"], left: Infix
   right,
 });
 
+export const makeFunctionExpression = (body: FunctionExpression["body"], parameter: FunctionExpression["parameter"] = []): FunctionExpression => ({
+  type: "function expression",
+  parameter,
+  body,
+});
+
 export const makeAssignment = (left: Assignment["left"], right: Assignment["right"]): Assignment => ({
   type: "assignment",
   left,