<script type="text/javascript"> 

function openwindow(url,name,iWidth,iHeight) 
{ 
// url 转向网页的地址  
// name 网页名称，可为空  
// iWidth 弹出窗口的宽度  
// iHeight 弹出窗口的高度  
//window.screen.height获得屏幕的高，window.screen.width获得屏幕的宽  
var iTop = (window.screen.height-30-iHeight)/2; //获得窗口的垂直位置;  
var iLeft = (window.screen.width-10-iWidth)/2; //获得窗口的水平位置;  
window.open(url,name,'height='+iHeight+',,innerHeight='+iHeight+',width='+iWidth+',innerWidth='+iWidth+',top='+iTop+',left='+iLeft+',toolbar=no,menubar=no,scrollbars=auto,resizeable=no,location=no,status=no'); 
} 

</script> 

<a href="javascript:void(0);" onclick="javascript:openwindow('a.html','',400,200);">转到a</a>