<style type="text/css">
#ddlEmployeeCars
{
display:none;
position:absolute;
top:50px;
left:9px;
}
</style>
<script language="javascript" type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>

　　 <asp:DropDownList ID="ddlEmployee" runat="server" AppendDataBoundItems="true">
<asp:ListItem Text="(Please Select)" Value="0" Selected="True" />
</asp:DropDownList>
<asp:DropDownList ID="ddlEmployeeCars" runat="server">
</asp:DropDownList>

<script language="javascript" type="text/javascript">
$(function() {
var $ddl = $("select[name$=ddlEmployee]");
var $ddlCars = $("select[name$=ddlEmployeeCars]");
$ddl.focus();
$ddl.bind("change keyup", function() {
if ($(this).val() != "0") {
loadEmployeeCars($("select option:selected").val());
$ddlCars.fadeIn("slow");
} else {
$ddlCars.fadeOut("slow");
}
});
});

function loadEmployeeCars(selectedItem) {
$.ajax({
type: "POST",
url: "Default.aspx/FetchEmployeeCars",
data: "{id: " + selectedItem + "}",
contentType: "application/json; charset=utf-8",
dataType: "json",
async: true,
success: function Success(data) {
printEmployeeCars(data.d);
}
});
}

function printEmployeeCars(data) {
$("select[name$=ddlEmployeeCars] > option").remove();
for (var i = 0; i < data.length; i++) {
$("select[name$=ddlEmployeeCars]").append(
$("<option></option>").val(data[i].Id).html(data[i].Car)
);
}
}
</script>

public partial class _Default : System.Web.UI.Page
{
[WebMethod]
public static List<EmployeeCar> FetchEmployeeCars(int id)
{
var emp = new EmployeeCar();
return emp.FetchEmployeeCars(id);
}

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
var employees = new Employee();
ddlEmployee.DataSource = employees.FetchEmployees();
ddlEmployee.DataTextField = "Surname";
ddlEmployee.DataValueField = "Id";
ddlEmployee.DataBind();
}
}
}

public class EmployeeCar
{
public int Id { get; set; }
public string Car { get; set; }

private static List<EmployeeCar> LoadData()
{
return new List<EmployeeCar>
{
new EmployeeCar {Id = 1, Car = "Ford"},
new EmployeeCar {Id = 1, Car = "Holden"},
new EmployeeCar {Id = 1, Car = "Honda"},
new EmployeeCar {Id = 2, Car = "Toyota"},
new EmployeeCar {Id = 2, Car = "General Motors"},
new EmployeeCar {Id = 2, Car = "Volvo"},
new EmployeeCar {Id = 3, Car = "Ferrari"},
new EmployeeCar {Id = 3, Car = "Porsche"},
new EmployeeCar {Id = 3, Car = "Ford2"}
};
}

public List<EmployeeCar> FetchEmployeeCars(int id)
{
return (from p in LoadData()
where p.Id == id
select p).ToList();
}
}

public class Employee
{
public int Id { get; set; }
public string GivenName { get; set; }
public string Surname { get; set; }

public List<Employee> FetchEmployees()
{
return new List<Employee>
{
new Employee {Id = 1, GivenName = "Tom", Surname = "Hanks"},
new Employee {Id = 2, GivenName = "Hugh", Surname = "Jackman"},
new Employee {Id = 3, GivenName = "Petter", Surname = "Liu"}
};
}

public Employee FetchEmployee(int id)
{
var employees = FetchEmployees();
return (from p in employees
where p.Id == id
select p).First();
}
}