Some of the built-in skins of the RadWindow have rounded corners, for example Black, Default, Forest, etc; most of the others have rounded corners at least for the titlebar. You can easily check them in this online demo
If none of the built-in skins matches your needs you would need to create a custom skin according to your preferences. How to do so is explained in this help article
. There is also information on the sprites which you will need to modify in order to have the radius you wish. Please note that if you change the size of the sprites you would need to basically rewrite the entire skin as sprites positions would not match then.
All the best,
the Telerik team
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