<asp:ListBox ID="ListBox1" runat="server">
<asp:ListItem Value="1">Add</asp:ListItem>
<asp:ListItem Value="2">Sub</asp:ListItem>
<asp:ListItem Value="3">Mul</asp:ListItem>
<asp:ListItem Value="4">Div</asp:ListItem>
</asp:ListBox>
<br />
<asp:Button ID="Button1" runat="server" Text="Button" />
protected void Button1_Click(object sender, EventArgs e)
{
ListItem l = new ListItem("Sub", "2");
ListBox1.SelectedIndex = ListBox1.Items.IndexOf(l);
}
Hope this code may help you.....