string postData = "user=123&pass=456"; // 要发放的数据 
byte[] byteArray = Encoding.UTF8.GetBytes(postData); 

HttpWebRequest objWebRequest = (HttpWebRequest)WebRequest.Create("http://www.abc.com/a.aspx"); 
objWebRequest.Method = "POST"; 
objWebRequest.ContentType = "application/x-www-form-urlencoded"; 
objWebRequest.ContentLength = byteArray.Length; 
Stream newStream = objWebRequest.GetRequestStream(); 
// Send the data. 
newStream.Write(byteArray, 0, byteArray.Length); //写入参数 
newStream.Close(); 

HttpWebResponse response = (HttpWebResponse)objWebRequest.GetResponse(); 
StreamReader sr=new StreamReader(response.GetResponseStream(), Encoding.Default); 
string textResponse = sr.ReadToEnd(); // 返回的数据

原文http://zhidao.baidu.com/question/89976700.html