diff --git a/src/study/day9Interface/TypeAssertions.go b/src/study/day9Interface/TypeAssertions.go
new file mode 100644
index 0000000..a30ffbf
--- /dev/null
+++ b/src/study/day9Interface/TypeAssertions.go
@@ -0,0 +1,31 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+func main() {
+	// 类型断言
+
+	var w io.Writer
+	w = os.Stdout
+	f := w.(*os.File) // success: f == os.Stdout
+	//c := w.(*bytes.Buffer) // panic: interface holds *os.File, not *bytes.Buffer
+	fmt.Println(f)
+	rw := w.(io.ReadWriter)
+	w = rw
+	fmt.Printf("(%T, %[1]v)\n", w)
+	// 被断言的接口值的动态类型不能是nil，否则会引发panic
+	w = nil
+	// panic: interface conversion: interface {} is nil, not io.ReadWriter
+	//rw = w.(io.ReadWriter) /
+	fmt.Printf("(%T, %[1]v)\n", w)
+
+	// 判断断言是否成功可以使用comma-ok断言
+	w = os.Stdout
+	f, ok := w.(*os.File) // success: f == os.Stdout, ok == true
+	fmt.Println(ok)
+	f.WriteString("hello, world") // 向控制台输出hello, world
+}