diff --git a/hello.go b/hello/hello.go
similarity index 100%
rename from hello.go
rename to hello/hello.go
diff --git a/hello_test.go b/hello/hello_test.go
similarity index 100%
rename from hello_test.go
rename to hello/hello_test.go
diff --git a/num/num.go b/num/num.go
new file mode 100644
index 0000000..c035ea3
--- /dev/null
+++ b/num/num.go
@@ -0,0 +1,13 @@
+package num
+
+func IsPrime(n int) bool {
+	if n <= 1 {
+		return false;
+	}
+	for i := 2; i < n; i++ {
+		if n % i == 0 {
+			return false;
+		}
+	}
+	return true;
+}
diff --git a/num/num_test.go b/num/num_test.go
new file mode 100644
index 0000000..52db523
--- /dev/null
+++ b/num/num_test.go
@@ -0,0 +1,24 @@
+package num
+
+import "testing"
+
+type checklist struct {
+	n int;
+	ret bool;
+}
+
+func TestNum(t *testing.T) {
+	list := []checklist{
+		{2, true},
+		{3, true},
+		{4, false},
+		{57, false},
+		{97, true},
+	}
+
+	for _, item := range list {
+		if item.ret != IsPrime(item.n) {
+			t.Errorf("n: %v faild, want %v", item.n, item.ret)
+		}
+	}
+}