C#中txt文本，中文寫入不覆蓋不亂碼

try

{
string path = @"E:\emr184\trunk\SinglePageTest\SinglePageTest\TXT\hello.txt";

string str = System.DateTime.Now.ToString() ;
FileStream fs = new FileStream(path,FileMode.Append);//文本加入不覆蓋

StreamWriter sw = new StreamWriter(fs,System.Text.Encoding.Default);//轉碼

sw.WriteLine(str);

//清空緩沖區
sw.Flush();
//關閉流
sw.Close();
 fs.Close();

}
catch (Exception)
{

　　Response.Write("<script>('寫入失敗！')</script>");
}