Fill rectangle with Rectangle.Fill tag and LinearGradientBrush
<UserControl x:Class='SilverlightApplication3.MainPage' xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml' xmlns:d='http://schemas.microsoft.com/expression/blend/2008' xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006' mc:Ignorable='d' d:DesignWidth='640' d:DesignHeight='480'> <Canvas> <Rectangle Canvas.Left = "40" Canvas.Top="40" Width="40" RadiusX="10" RadiusY="10"> <Rectangle.Fill> <LinearGradientBrush> <GradientStop Offset="0" Color="Blue"/> <GradientStop Offset=".8" Color="Yellow"/> </LinearGradientBrush> </Rectangle.Fill> </Rectangle> </Canvas> </UserControl>