用伪类清除浮动

<ul class="clear">

      <li><img src="images/img_01.jpg"></li>

      <li><img src="images/img_02.jpg"></li>

      <li><img src="images/img_03.jpg"></li>

      <li><img src="images/img_04.jpg"></li>

</ul>

.clear{zoom:1;}

.clear:after{display:block; content:""; height: 0; clear:both; overflow:hidden; visibility:hidden;}