RadAjaxManager allows a single instance of the control on the same page only. You can use RadAjaxManagerProxy when using usercontrols. Check the following help documentation which explains the same. RadAjax and WebUserControls
Thanks for Reply,
I aware of these basics.
I tried to do Postback on UserControl, whcih cannot be done because Asp.net is not able to for exact ClientID for the DIV which is representing that usercontrol. I passed uniqueID of UserControl, I got exception, i digged into Javascript i found above one as reason.
I used Radajax Panel and it worked fine.