Ajax calling is not working. Any one may help me...

$('.del').click(function (event) {
                var parentID = { "CategoryID": $(this).closest('li').attr('id') };
                parentID = JSON.stringify(parentID);

                alert('before ajax calling');
                $.ajax({
                    type: "POST",
                    url: "MasterService.asmx/RemoveCategory",
                    data: parentID,
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    success: function (msg) {
                        alert('hurray! Success. ' + msg.d); $(this).closest('li').remove();
                    },
                    failure: function (response) {
                        alert(response.d);
                    }
                });


            });

[WebMethod]
public string RemoveCategory(string CategoryID)
{
    CategoryMasterBAL categorymasterBAL = new CategoryMasterBAL();

    return categorymasterBAL.RemoveCategory(CategoryID);
}