Hi Sandeep,
You can use the ExpanderStyle property of the RadTreeView to set the style of the expander. Basically the expander in RadTreeView is a ToggleButton. You can see example how to use the expander property here:
http://demos.telerik.com/silverlight/#Examples/TreeView/ExpanderStyle
Please let us know if you have anny other questions.
All the best,
Valentin.Stoychev
the Telerik team
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