解决马上给分...

var oFrame = document.getElementById("mainFrame");
switch(id) {
  case "receives":
    oFrame.src="MessageIndex.aspx";
    break;
  case "sended":
    oFrame.src="Outbox.aspx";
    break;
  case "deleted":
    oFrame.src="Deleted.aspx";
    break;
  case "send":
    oFrame.src="MessageIndex.aspx";
    break;
  default:
    break;
}