<Canvas x:Name="root" 
  xmlns="http://schemas.microsoft.com/client/2007"  
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
  Width="500" Height="70">
  <Rectangle x:Name="rect" Canvas.Top="10" Canvas.Left="400" Width="50" Height="50" Fill="Green">                                                                      
    <Rectangle.Triggers>
      <EventTrigger  RoutedEvent="Rectangle.Loaded" >
       <BeginStoryboard>
        <Storyboard BeginTime="0:0:1">
          <DoubleAnimation 
            Storyboard.TargetName="rect"
            Storyboard.TargetProperty="(Canvas.Left)"
            From="10"
            Duration="00:00:03"
          />
        </Storyboard>
       </BeginStoryboard>
     </EventTrigger>
    </Rectangle.Triggers>
  </Rectangle>
</Canvas>

