Fill rectangle with Rectangle.Fill tag and LinearGradientBrush
<Window x:Class="WpfApplication1.ShapesWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="ShapesWindow" Height="160" Width="400"> <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> </Window>