If you want your rectangle to have rounded edges, you can use the RadiusX and RadiusY properties of the Rectangle control.
Xaml
<Rectangle x:Name="MyRect" Width="100" Height="100" RadiusX="10" RadiusY="10" Fill="Red" />
RadiusX determines the radius of the corners on the x-axis and RadiusY determines the radius of the corners on the y-axis.
That's it!