<Canvas x:Name="LayoutRoot">
 <Canvas.Resources>
  <Storyboard x:Name="MyStoryboard">
   <DoubleAnimationUsingKeyFrames Storyboard.TargetName="rect" Storyboard.TargetProperty="Width">
    <SplineDoubleKeyFrame KeyTime="00:00:01" Value="200" KeySpline="1.0,0.0,1.0,1.0" />
    <SplineDoubleKeyFrame KeyTime="00:00:05" Value="300" KeySpline="0.0,1.0,1.0,1.0" />
   </DoubleAnimationUsingKeyFrames>
  </Storyboard>
 </Canvas.Resources>
 <Rectangle x:Name="rect" Width="100" Height="100" Canvas.Left="0" Canvas.Top="0">
  <Rectangle.Fill>
   <SolidColorBrush Color="Red" />
  </Rectangle.Fill>
 </Rectangle>
</Canvas>

View : http://myifu.com/_study/silverlight/template_animation_12_usingkeyframe02/