<script type="text/javascript">
function Validate() {
var ddl = document.getElementById('<%=DropDownList1.ClientID%>');
var option = ddl.options[ddl.selectedIndex];
alert(option.value);
if (option.value == 0) {
alert('select value from dropdown');
return false;
}
else {
return true;
}
}
</script>
<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="return Validate();" />