File.WriteAllBytes doesn't work, just write 0 bytes to the file...(android)

hi guys,

I am new to unity..just try to read and write file in android, and it just read fine but writing failed. I have try to put the file in "Plugins/Android/Assets" or "StreamingAsstes", any way it read fine, just can't write to the persistentDataPath , or say, worte but 0 bytes!!

my code(very simple) :

         // i just don't care the file exists or not, just write 

      string filepath = Application.persistentDataPath + "/" + "testword.bytes";

      WWW loadDB = new WWW("jar:file://" + Application.dataPath + "!/assets/" + "testword.bytes");  

       //GameObject.Find("Canvas").transform.GetChild(0).GetComponent<Text>().text = "read failed ";

       while (!loadDB.isDone) {; }  //

         GameObject.Find("Canvas").transform.GetChild(0).GetComponent<Text>().text = loadDB.text+ "read OK";
   /*========untill here, everything is allright, and my Text shows the content and read OK. ===*/
         // then save to Application.persistentDataPath
         try
         {
             System.IO.File.WriteAllBytes(filepath, loadDB.bytes);  //<<<I checked the new file, it just 0 bytes!!
         }
         catch (System.Exception e)
         {
             Debug.LogException(e, this);
             GameObject.Find("Canvas").transform.GetChild(0).GetComponent<Text>().text = "error**"+ e.ToString();
         }

         /*========no exception , but, when I check the file , there is 0 bytes in it.=====*/

Thank you guys, it makes me so crazy (cause I am using Sqlite..simpleSql, it just cannot load any thing)