I am trying to set padding of a StackPanel but there ain't such property. I tried StackPanel.Border, but there is no such property either.
Any ideas?
You could put a Border around the StackPanel and set a padding on that. I end up doing this a lot, since there are many UIElements that do not have a padding property.
<Border Padding="10">
<StackPanel>
<!--...-->
</StackPanel>
</Border>
(Note: all FrameworkElements have a Margin property, which will space the element, but not include the margin width as part of the ActualWidth).
If you want to space the items inside a StackPanel, you'll want to add a margin to each child as Rob said.