您可以设置MetroWindow的TitleTemplate属性。

<Controls:MetroWindow.TitleTemplate>
    <DataTemplate>
        <TextBlock Text="{TemplateBinding Content}"
                   TextTrimming="CharacterEllipsis"
                   VerticalAlignment="Center"
                   Margin="8 -1 8 0"
                   FontWeight="Light"
                   FontSize="{DynamicResource WindowTitleFontSize}"
                   FontFamily="{DynamicResource HeaderFontFamily}" />
    </DataTemplate>
</Controls:MetroWindow.TitleTemplate>

或用大写字母表示标题：

<Controls:MetroWindow.TitleTemplate>
    <DataTemplate>
        <TextBlock Text="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=Content, Converter={Converters:ToUpperConverter}}"
                   TextTrimming="CharacterEllipsis"
                   VerticalAlignment="Center"
                   Margin="8 -1 8 0"
                   FontWeight="Light"
                   FontSize="{DynamicResource WindowTitleFontSize}"
                   FontFamily="{DynamicResource HeaderFontFamily}" />
    </DataTemplate>
</Controls:MetroWindow.TitleTemplate>

为了更新@punker76的答案，在2.x版中，资源被重命名，因此示例如下

<mah:MetroWindow.TitleTemplate>
   <DataTemplate>
      <TextBlock Text="{TemplateBinding Content}"
                 TextTrimming="CharacterEllipsis"
                 VerticalAlignment="Center"
                 Margin="8 -1 8 0"
                 FontWeight="Normal"
                 FontSize="{DynamicResource MahApps.Font.Size.Window.Title}"
                 FontFamily="{DynamicResource MahApps.Fonts.Family.Window.Title}" />
   </DataTemplate>
</mah:MetroWindow.TitleTemplate>