Hi Manishkumar,
Check out this forum post : http://www.telerik.com/community/forums/silverlight/chart/annotation-text.aspx
I have included a project where I retemplate marked zone to show text.
I hope this gets you started properly.
Best wishes,
Evgeni "Zammy" Petrov
the Telerik team
Browse the
videos here>> to help you get started with RadControls for Silverlight