Hello Manoj,
This one is very hard to tell. Are you using some sort of caching? Or maybe the browser uses GET caching (so you need to add time stamp to the URL you are opening with javascript). Where are you binding the grid - are you using InitializeDataSource, etc.
It is really tough to tell without seeing the implmentation details first. By the way, I can recommend using our own WebDialogWindow component instead of window.open - much better results (better interface, better programmatic control over the window, skins, etc) - you can see WebDialogWindow in action here: