diff --git a/CHANGELOG.md b/CHANGELOG.md
index 03e26af..65b2954 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,17 @@
 # @greenlabs/rescript-korean-numeral
 
+## 0.4.0
+
+### Breaking Changes
+
+- ReScript v11.1 BigInt 지원
+- fromInt64 API 제거
+- fromString 함수의 반환 타입 변경: `string` -> `option<string>`
+
+## 0.3.0
+
+- ReScript v11 지원
+
 ## 0.2.0
 
 ### Minor Changes
diff --git a/__tests__/KoreanNumeral_test.res b/__tests__/KoreanNumeral_test.res
index 14723cb..46d392b 100644
--- a/__tests__/KoreanNumeral_test.res
+++ b/__tests__/KoreanNumeral_test.res
@@ -4,15 +4,15 @@ describe(`1만 이하 표시`, () => {
   open Expect
 
   test(`KoreanNumeral.fromInt`, () => {
-    let result = KoreanNumeral.fromInt(1234, ())
+    let result = KoreanNumeral.fromInt(1234)
     expect(result)->toBe(`1,234`)
   })
   test(`KoreanNumeral.fromFloat`, () => {
-    let result = KoreanNumeral.fromFloat(152212525110.0, ())
+    let result = KoreanNumeral.fromFloat(152212525110.0)
     expect(result)->toBe(`1,522억 1,252만 5,110`)
   })
-  test(`KoreanNumeral.fromInt64`, () => {
-    let result = KoreanNumeral.fromInt64(Int64.of_string("1234567890987654321"), ())
+  test(`KoreanNumeral.fromBigInt`, () => {
+    let result = KoreanNumeral.fromBigInt(1234567890987654321n)
     expect(result)->toBe(`123경 4,567조 8,909억 8,765만 4,321`)
   })
 })
@@ -21,35 +21,51 @@ describe(`1만 이하는 절삭`, () => {
   open Expect
 
   test(`1234 => ""`, () => {
-    let result = KoreanNumeral.fromInt(1234, ~drop=1, ())
+    let result = KoreanNumeral.fromInt(1234, ~drop=1)
     expect(result)->toBe(``)
   })
   test(`1234567 => "123만"`, () => {
-    let result = KoreanNumeral.fromInt(1234567, ~drop=1, ())
+    let result = KoreanNumeral.fromInt(1234567, ~drop=1)
     expect(result)->toBe(`123만`)
   })
   test(`1234567890 => "12억 3,456만"`, () => {
-    let result = KoreanNumeral.fromInt(1234567890, ~drop=1, ())
+    let result = KoreanNumeral.fromInt(1234567890, ~drop=1)
     expect(result)->toBe(`12억 3,456만`)
   })
   test(`1234567890.1 => "12억 3,456만"`, () => {
-    let result = KoreanNumeral.fromFloat(1234567890.1, ~drop=1, ())
+    let result = KoreanNumeral.fromFloat(1234567890.1, ~drop=1)
     expect(result)->toBe(`12억 3,456만`)
   })
   test(`152212525110 => "1,522억 1,252만"`, () => {
-    let result = KoreanNumeral.fromFloat(152212525110.0, ~drop=1, ())
+    let result = KoreanNumeral.fromFloat(152212525110.0, ~drop=1)
     expect(result)->toBe(`1,522억 1,252만`)
   })
   test(`152212525110 => "1,522억"`, () => {
-    let result = KoreanNumeral.fromFloat(152200005110.0, ~drop=1, ())
+    let result = KoreanNumeral.fromFloat(152200005110.0, ~drop=1)
     expect(result)->toBe(`1,522억`)
   })
   test(`220000152212525110.0 => "22경 1,522억 1,252만"`, () => {
-    let result = KoreanNumeral.fromFloat(220000152212525110.0, ~drop=1, ())
+    let result = KoreanNumeral.fromFloat(220000152212525110.0, ~drop=1)
     expect(result)->toBe(`22경 1,522억 1,252만`)
   })
   test(`1234567890987654321 => "123경 4,567조 8,909억 8,765만"`, () => {
-    let result = KoreanNumeral.fromString("1234567890987654321", ~drop=1, ())
+    let result = KoreanNumeral.fromString("1234567890987654321a", ~drop=1)
+    expect(result)->toBe(None)
+  })
+  test(`152212525110n => "1,522억 1,252만"`, () => {
+    let result = KoreanNumeral.fromBigInt(152212525110n, ~drop=1)
+    expect(result)->toBe(`1,522억 1,252만`)
+  })
+  test(`152212525110n => "1,522억"`, () => {
+    let result = KoreanNumeral.fromBigInt(152200005110n, ~drop=1)
+    expect(result)->toBe(`1,522억`)
+  })
+  test(`220000152212525110n => "22경 1,522억 1,252만"`, () => {
+    let result = KoreanNumeral.fromBigInt(220000152212525110n, ~drop=1)
+    expect(result)->toBe(`22경 1,522억 1,252만`)
+  })
+  test(`1234567890987654321n => "123경 4,567조 8,909억 8,765만"`, () => {
+    let result = KoreanNumeral.fromBigInt(1234567890987654321n, ~drop=1)
     expect(result)->toBe(`123경 4,567조 8,909억 8,765만`)
   })
 })
diff --git a/src/KoreanNumeral.res b/src/KoreanNumeral.res
index 954d643..7d28906 100644
--- a/src/KoreanNumeral.res
+++ b/src/KoreanNumeral.res
@@ -3,17 +3,21 @@
 open Belt
 
 let unitsInKorean = list{``, `만`, `억`, `조`, `경`}
-let mann = Int64.of_int(10000)
+let mann = 10000n
 
-let rec split = divided =>
-  Int64.equal(divided, Int64.zero)
-    ? list{}
-    : list{Int64.rem(divided, mann), ...split(Int64.div(divided, mann))}
+let rec split = divided => {
+  open! Js.BigInt
+  if divided == 0n {
+    list{}
+  } else {
+    list{Js.BigInt.mod(divided, mann), ...split(divided / mann)}
+  }
+}
 
-let fromInt64 = (num, ~drop=0, ()) => {
+let fromBigInt = (num, ~drop=0) => {
   split(num)
   ->List.zipBy(unitsInKorean, (num, unit) => {
-    Int64.equal(num, Int64.zero) ? "" : num->Int64.to_int->toLocaleString("ko-KR") ++ unit
+    num == 0n ? "" : num->Js.BigInt.toLocaleString ++ unit
   })
   ->List.drop(drop)
   ->Option.getWithDefault(list{})
@@ -23,14 +27,18 @@ let fromInt64 = (num, ~drop=0, ()) => {
   ->(Js.Array.joinWith(" ", _))
 }
 
-let fromInt = (num, ~drop=0, ()) => {
-  fromInt64(Int64.of_int(num), ~drop, ())
+let fromInt = (num, ~drop=0) => {
+  num->Int.toString->Js.BigInt.fromStringExn->fromBigInt(~drop)
 }
 
-let fromFloat = (num, ~drop=0, ()) => {
-  fromInt64(Int64.of_float(ceil(num)), ~drop, ())
+let fromFloat = (num, ~drop=0) => {
+  num->ceil->Float.toString->Js.BigInt.fromStringExn->fromBigInt(~drop)
 }
 
-let fromString = (num, ~drop=0, ()) => {
-  fromInt64(Int64.of_string(num), ~drop, ())
+let fromString = (num, ~drop=0) => {
+  try {
+    num->Js.BigInt.fromStringExn->fromBigInt(~drop)->Some
+  } catch {
+  | _ => None
+  }
 }