Set TextWrapping to True for TextBlock
<Window x:Class="WpfApplication1.Window1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="LocalInstallFun" > <TextBlock FontSize="48" TextWrapping="Wrap">this is a test.</TextBlock> </Window>