diff --git a/25_1000_digit_fibonacci_number/main.go b/25_1000_digit_fibonacci_number/main.go
new file mode 100644
index 0000000..8398574
--- /dev/null
+++ b/25_1000_digit_fibonacci_number/main.go
@@ -0,0 +1,40 @@
+package main
+
+import (
+	"fmt"
+	"math/big"
+)
+
+func main() {
+	n := 11
+	for {
+		fb := fibonacci(n)
+		fmt.Println("index =", n, "len =", len(fb.String()))
+		if numberDigits(fb) >= 1000 {
+			break
+		}
+		n++
+	}
+
+	fmt.Println("n is", n)
+}
+
+func fibonacci(index int) *big.Int {
+	if index < 3 {
+		ret := (&big.Int{}).SetUint64(1)
+		return ret
+	}
+	fn := &big.Int{}
+	fn1 := (&big.Int{}).SetInt64(1)
+	fn2 := (&big.Int{}).SetInt64(1)
+	for i := 3; i <= index; i++ {
+		fn.Set(fn.Add(fn1, fn2))
+		fn2.Set(fn1)
+		fn1.Set(fn)
+	}
+	return fn
+}
+
+func numberDigits(num *big.Int) int {
+	return len(num.String())
+}
diff --git a/25_1000_digit_fibonacci_number/main_test.go b/25_1000_digit_fibonacci_number/main_test.go
new file mode 100644
index 0000000..f4507f2
--- /dev/null
+++ b/25_1000_digit_fibonacci_number/main_test.go
@@ -0,0 +1,46 @@
+package main
+
+import (
+	"math/big"
+	"reflect"
+	"testing"
+)
+
+func Test_fibonacci(t *testing.T) {
+	type args struct {
+		index int
+	}
+	tt := []struct {
+		name string
+		args args
+		want *big.Int
+	}{
+		{
+			"1",
+			args{1},
+			(&big.Int{}).SetBits([]big.Word{1}),
+		},
+		{
+			"10",
+			args{10},
+			(&big.Int{}).SetBits([]big.Word{big.Word(55)}),
+		},
+		{
+			"12",
+			args{12},
+			(&big.Int{}).SetBits([]big.Word{big.Word(144)}),
+		},
+		{
+			"13",
+			args{13},
+			(&big.Int{}).SetBits([]big.Word{big.Word(233)}),
+		},
+	}
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := fibonacci(tc.args.index); !reflect.DeepEqual(got, tc.want) {
+				t.Errorf("fibonacci() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}