1. 方法1

<Separator BorderBrush="Gray">
	<Separator.LayoutTransform>
		<RotateTransform Angle="90" />
  	</Separator.LayoutTransform>
</Separator>

2. 方法2

<Separator Style="{StaticResource {x:Static ToolBar.SeparatorStyleKey}}" 
		BorderBrush="Gray" />

参考

WPF如何制作一个竖直的separator