Set pen for TextDecorations
<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" HorizontalAlignment="Center" VerticalAlignment="Center" TextElement.FontFamily="Palatino Linotype" FontSize="40"> <Span> <Span.TextDecorations> <TextDecoration Location="StrikeThrough"> <TextDecoration.Pen> <Pen Brush="LightGreen" Thickness="2" /> </TextDecoration.Pen> </TextDecoration> </Span.TextDecorations> this is a test </Span> </Window>