This way
<asp:ListBox ID="ListBox1" runat="server" Width="150" Height="60" SelectionMode="Multiple">
<asp:ListItem Text="Mango" Value="1"></asp:ListItem>
<asp:ListItem Text="Apple" Value="2"></asp:ListItem>
<asp:ListItem Text="Banana" Value="3"></asp:ListItem>
<asp:ListItem Text="Guava" Value="4"></asp:ListItem>
<asp:ListItem Text="Pineapple" Value="5"></asp:ListItem>
<asp:ListItem Text="Papaya" Value="6"></asp:ListItem>
<asp:ListItem Text="Grapes" Value="7"></asp:ListItem>
</asp:ListBox>
<br />
<hr />
<asp:Button ID="btnGetValues" Text="Get values" runat="server" OnClientClick="return GetValues()" />
<script type="text/javascript">
function GetValues() {
var values = "";
var listBox = document.getElementById("<%= ListBox1.ClientID%>");
for (var i = listBox.options.length - 1; i >= 0; i--) {
if (listBox.options[i].selected) {
values += listBox.options[i].innerHTML + " " + listBox.options[i].value + "\n";
}
}
alert(values);
return false;
}
</script>