<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"
    Title="Window1" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d">
    <Grid>
        <telerik:RadGridView x:Name="RadGridView1">
            <telerik:RadGridView.Resources>
                <Style TargetType="telerik:GridViewHeaderCell">
                    <Setter Property="LayoutTransform">
                        <Setter.Value>
                            <RotateTransform Angle="-90"/>
                        </Setter.Value>
                    </Setter>
                </Style>
            </telerik:RadGridView.Resources>
        </telerik:RadGridView>
    </Grid>
</Window>
