<div id="topBox">
  <div id="top">
    <ul id="navMain">
      <li><a href="#">logo_img</a> </li>
      <li class="story"><a href="#">1</a>
        <div class="navSubBox">
          <ul class="navSub">
            <li><a class="curNav" href="#">11</a> <span class="clear"></span> </li>
            <li><a href="#">12</a></li>
          </ul>
        </div>
      </li>
      <li class="dongtai"><a href="#">3</a>
        <div class="navSubBox">
          <ul class="navSub">
            <li><a class="curNav" href="#">31</a> <span class="clear"></span> </li>
            <li><a href="#">32</a></li>
          </ul>
        </div>
      </li>
      <li><a href="#">3</a></li>
      <li><a href="#">4</a></li>
      <li><a href="#">5</a></li>
      <li><a href="#">6</a></li>
      <li><a href="#">7</a></li>
    </ul>
    <div class="clear"></div>
  </div>
</div>

#topBox { height:30px; position:relative;z-index:20;}
#top { width:1004px;background:blue;margin:0 auto; height:30px;}

ul#navMain { float:left;}
#navMain li { float:left; display:block; }
#navMain li a { width:105px; display:block; text-align:center;  color:#dadada;}

.navSubBox{position:absolute; left:0;top:30px; _top:32px; width:100%; _width:expression(documentElement.offsetWidth -19); background:#ccc;/*display:none;*/z-index:20;}
.navSub{ margin:0 auto;overflow:hidden;height:33px;line-height:33px;padding-left:240px;}
.navSub li{ float:left;display:block;}
#topBox .navSub li a{height:33px;line-height:33px;padding:0; margin:0;color:#000;}
.clear {clear:both;height:0;font-size:0px;}