.backToTop {

    display: none;

    width: 18px;

    line-height: 1.2;

    padding: 5px 0;

    background-color: #000;

    color: #fff;

    font-size: 12px;

    text-align: center;

    position: fixed;

    _position: absolute;

    right: 10px;

    bottom: 100px;

    _bottom: "auto";

    cursor: pointer;

    opacity: .6;

    filter: Alpha(opacity=60);

}

JS代码：

(function() {

    var $backToTopTxt = "返回顶部", $backToTopEle = $('<div class="backToTop"></div>').appendTo($("body"))

        .text($backToTopTxt).attr("title", $backToTopTxt).click(function() {

            $("html, body").animate({ scrollTop: 0 }, 120);

    }), $backToTopFun = function() {

        var st = $(document).scrollTop(), winh = $(window).height();

        (st > 0)? $backToTopEle.show(): $backToTopEle.hide();

        //IE6下的定位

        if (!window.XMLHttpRequest) {

            $backToTopEle.css("top", st + winh - 166);

        }

    };

    $(window).bind("scroll", $backToTopFun);

    $(function() { $backToTopFun(); });

})();