<div style="display:none" id="goTopBtn"><img src="http://www.unitymanual.com/m/Manual/img/top.png" width="54" height="54" border="0"></div>
<script type="text/javascript">

function goTopEx(){

        var obj=document.getElementById("goTopBtn");

        function getScrollTop(){

                return document.documentElement.scrollTop || document.body.scrollTop

            }

        function setScrollTop(value){

				document.documentElement.scrollTop = value;

   				document.body.scrollTop = value;

            }

        window.onscroll=function(){getScrollTop()>0?obj.style.display="":obj.style.display="none";}

        obj.onclick=function(){

            var goTop=setInterval(scrollMove,10);

            function scrollMove(){

                    setScrollTop(getScrollTop()/1.1);

                    if(getScrollTop()<1)clearInterval(goTop);

                }

        }

    }

goTopEx();
实现2：
 
#scrolltop{
    visibility:hidden;
    position:fixed;
    bottom:100px;
    display:block;
    margin:-30px 0 0 2px;
    width:40px;
    background:#f4f4f4;
    border:1px #cdcdcd solid;
    border-radius:3px;
    border-top:0;
    cursor:pointer;
}

#scrolltop:hover{
    text-decoration:none;
}

.ie6 #scrolltop{
    position:absolute;
    bottom:auto;
}

#scrolltop a{
    display:block;
    width:30px;
    height:24px;
    padding:3px 5px;
    line-height:12px;
    text-align:center;
    color:#787878;
    text-decoration:none;
    background:url(http://www.unitymanual.com/static/image/common/scrolltop.png) no-repeat 0 0;
    border-top:1px #cdcdcd solid;
}

a.scrolltopa:hover{
    background-position:-40px 0px !important;
}

a.replyfast{
    background-position:0 -30px !important;
}

a.replyfast:hover{
    background-position:-40px -30px !important;
}

a.returnlist,a.returnboard{
    background-position:0 -60px !important;
}

a.returnlist:hover,a.returnboard:hover{
    background-position:-40px -60px !important;
}

#scrolltop a b{
    visibility:hidden;
    font-weight:normal;
}

<div style="left: auto; right: 0px; visibility: visible;" id="scrolltop"> 
   
  <span hidefocus="true"><a title="返回顶部" onclick="window.scrollTo('0','0')" class="scrolltopa"><b>返回顶部</b></a></span> 
   
</div>