Ajax和ActiveXObject

why when I run this code on local server, req.send(sData) doesn't work? It runs alert1 but doesn't run alert2;

function initialize() {
    var req;
    try {
        req = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
        try {
            req = new ActiveXObject("Microsoft.XMLHTTP");
        }
        catch (oc) {
            req = null;
        }
    }
    if (!req && typeof XMLHttpRequest != "undefined") {
        req = new XMLHttpRequest();
    }
    return req;
}

function getData(sUrl, sData) {
    var req = initialize();
    req.open("POST", sUrl, false);
    req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
    alert("1");
    req.send(sData);
    alert("2");
    return req.responseText;
}

sUrl and sData are the same as the original website sends.