<div id="div1">

    </div>

<style type="text/css">
      #div1 {
        width: 200px;
        height: 200px;
        background: red;
        position: absolute;
      }
    </style>

<script type="text/javascript">
   window.onload = function() {
    var oDiv = document.getElementById("div1");
    var disX = 0;
    var disY = 0;
    oDiv.onmousedown = function(ev) {
     var oEvent = ev || event; // 浏览器兼容
     disX = oEvent.clientX - oDiv.offsetLeft; // 横向的位置就是鼠标的位置-div的位置
     disY = oEvent.clientY - oDiv.offsetTop;
    };

    oDiv.onmousemove = function(ev) {
      var oEvent = ev || event;
      oDiv.style.left = oEvent.clientX - disX+'px'; // 当前鼠标的位置-disX
      oDiv.style.top = oEvent.clientY - disY+'px';
     };
   };
  </script>

<script type="text/javascript">
   window.onload = function() {
    var oDiv = document.getElementById("div1");
    var disX = 0;
    var disY = 0;
    oDiv.onmousedown = function(ev) {
     var oEvent = ev || event; // 浏览器兼容
     disX = oEvent.clientX - oDiv.offsetLeft; // 横向的位置就是鼠标的位置-div的位置
     disY = oEvent.clientY - oDiv.offsetTop;

     oDiv.onmousemove = function(ev) {
      var oEvent = ev || event;
      oDiv.style.left = oEvent.clientX - disX+'px'; // 当前鼠标的位置-disX
      oDiv.style.top = oEvent.clientY - disY+'px';
     };
     oDiv.onmouseup = function() {
      oDiv.onmousemove = null;
      oDiv.onmouseup = null; // 不留垃圾,鼠标抬起本来也就没有用了
     };

    };

   };
  </script>