// ajax 对象

function ajaxObject() {

    var xmlHttp;

    try {

        // Firefox, Opera 8.0+, Safari

        xmlHttp = new XMLHttpRequest();

        }

    catch (e) {

        // Internet Explorer

        try {

                xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");

            } catch (e) {

            try {

                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");

            } catch (e) {

                alert("您的浏览器不支持AJAX！");

                return false;

            }

        }

    }

    return xmlHttp;

}

// ajax post请求：

function ajaxPost ( url , data , fnSucceed , fnFail , fnLoading ) {

    var ajax = ajaxObject();

    ajax.open( "post" , url , true );

    ajax.setRequestHeader( "Content-Type" , "application/x-www-form-urlencoded" );

    ajax.onreadystatechange = function () {

        if( ajax.readyState == 4 ) {

            if( ajax.status == 200 ) {

                fnSucceed( ajax.responseText );

            }

            else {

                fnFail( "HTTP请求错误！错误码："+ajax.status );

            }

        }

        else {

            fnLoading();

        }

    }

    ajax.send( data );

}

<html xmlns="http://www.w3.org/1999/xhtml">

<head>

<title></title>

<script src="Scripts/jquery-1.4.1.min.js" type="text/javascript"></script>

<script type="text/javascript">

$().ready(function () {

　　$('#selectNum').change(function () {

　　　　var idValue = $(this).val();

　　　　//采用POST方式调用服务

　　　　$.post('Server.aspx', { id: idValue }, function (text, status) { alert(text); });

　　})

})

</script>

</head>

<body>

<select id="selectNum">

　　<option value="0">--Select--</option>

　　<option value="1">1</option>

　　<option value="2">2</option>

　　<option value="3">3</option>

</select>

</body>

</html>