a id="_top"></a>

<script type="text/javascript">
function form_top(){
document.write('<div id="form_top"><a href="#_top" title="回到頂部"><div class="top_img"></div></a></div>')
}
form_top();
</script>

#form_top{display:block; bottom:3px; right:3px; position:fixed;}
.top_img{
background-image: url("img/top.png");
width: 40px;
height:40px;
cursor: pointer;
}