Use Run to control the font style for TextBlock
<UserControl x:Class='SilverlightApplication3.MainPage'
xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'
xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
xmlns:d='http://schemas.microsoft.com/expression/blend/2008'
xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006'
mc:Ignorable='d'
d:DesignWidth='640'
d:DesignHeight='480'>
<StackPanel x:Name="LayoutRoot" Background="White" >
<TextBlock FontFamily="Georgia" FontSize="20" Margin="15" TextWrapping="Wrap" LineStackingStrategy="BlockLineHeight" LineHeight="20">
this is a test
<Run FontFamily="Comic Sans MS" Foreground="Red" FontSize="40">test</Run>,
</TextBlock>
<TextBlock FontFamily="Georgia" FontSize="20" Margin="15" TextWrapping="Wrap" LineStackingStrategy="MaxHeight">
this is a test
<Run FontFamily="Comic Sans MS" Foreground="Red" FontSize="40">MaxHeight</Run>,
</TextBlock>
</StackPanel>
</UserControl>
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