Ajax请求使用post

I am using simple javascript to post a form via ajax.When i use get everything works well , but as it is get request all the parameters are visible. With post this is my code but when i use it the request still uses get , any suggestions to fix it.

      var url = url;
      var params = params;



      xmlhttp.onreadystatechange=cfunc;
      xmlhttp.open("POST",url,true);


     xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
     xmlhttp.setRequestHeader("Content-length", params.length);
     xmlhttp.setRequestHeader('X_REQUESTED_WITH','XMLHttpRequest');  
     xmlhttp.setRequestHeader("Connection", "close"); 
     xmlhttp.send(params); 

xmlhttp.open('POST', URL, true); is the correct form. What is telling you that it's GET?

This is an AJAX request, that shows as post for me, so you can just grab that if you want: http://jsfiddle.net/robert/AUkt4/