#backtotop {
width:100px;
background:#F4FFBF;
border:1px solid #ccc;
text-align:center;
padding:5px;
position:fixed;
bottom:10px;right:10px;
cursor:pointer;
color:#666;
text-decoration:none;
}

<a href="#" id="backtotop">Lên đầu trang</a>

<script src='http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js' type='text/javascript'></script>

<script type='text/javascript'>

$(function(){$.fn.scrollToTop=function(){$(this).hide().removeAttr("href");if($(window).scrollTop()!="0"){$(this).fadeIn("slow")}var scrollDiv=$(this);$(window).scroll(function(){if($(window).scrollTop()=="0"){$(scrollDiv).fadeOut("slow")}else{$(scrollDiv).fadeIn("slow")}});$(this).click(function(){$("html, body").animate({scrollTop:0},"slow")})}});



$(function() {

$("#toTop").scrollToTop();

});

</script>