How do I make comboboxitem rounded corner in WPF?

What you need to do is override your style. As I can't really visualise what you're trying to achieve with this, I'll give you an example where only the selected item has a rounded border, and the background is set to green. You should really have a play around with something like this:

<Window x:Class="RoundedComboBoxItem.MainWindow"
        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"
        xmlns:local="clr-namespace:RoundedComboBoxItem"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800"
  xmlns:theme="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero2">

  <Window.Resources>
    <Style 
       TargetType="{x:Type ComboBoxItem}">
      <Setter Property="SnapsToDevicePixels"
          Value="true" />
      <Setter Property="OverridesDefaultStyle"
          Value="true" />
      <Setter Property="Foreground" Value="Red" />
      <Setter Property="Template">
        <Setter.Value>
          <ControlTemplate TargetType="{x:Type ComboBoxItem}">
            <Border x:Name="Border"
                Padding="2"
                SnapsToDevicePixels="true"
                Background="Transparent"
                    CornerRadius="8"
                >
              <VisualStateManager.VisualStateGroups>
                <VisualStateGroup x:Name="SelectionStates">
                  <VisualState x:Name="Unselected" />
                  <VisualState x:Name="Selected">
                    <Storyboard>
                      <ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
                                                Storyboard.TargetProperty="(Panel.Background).
                    (SolidColorBrush.Color)">
                        <EasingColorKeyFrame KeyTime="0"
                                         Value="Green" />
                      </ColorAnimationUsingKeyFrames>
                    </Storyboard>
                  </VisualState>
                </VisualStateGroup>
              </VisualStateManager.VisualStateGroups>
              <ContentPresenter />
            </Border>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>
  </Window.Resources>

  <Grid>
    <ComboBox HorizontalAlignment="Left" Margin="154,133,0,0" VerticalAlignment="Top" Width="372">
      <ComboBoxItem Content="Item No. 23" />
      <ComboBoxItem Content="Item No. 24"/>
      <ComboBoxItem Content="Item No. 25">
      </ComboBoxItem>
    </ComboBox>

  </Grid>
</Window>