Hi Muhammad Djamaluddin,
You will need to edit the control template of the RadOutollBarItems in Blend and remove the OuterBorder_Selected BorderBrush from the SelectVisual Border.
"-1 0 -1 -1"
I prepared an example illustrating this approach. Please take a look at it and let me know if it works for you or if you need more info.
Also you can find more information on How to edit control templates in Expression Blend here
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