设置样式不对齐呢

 <div class="nav">
				<div class="nav_left">
				</div>
				<div class="nav_mid">
					<div class="nav_mid_left">
						<ul>
							<li><a href="#">首页</a></li>
							<li><a href="list.html">关于慕课</a></li>
							<li><a href="list.html">新闻动态</a></li>
							<li><a href="list.html">课程中心</a></li>
							<li><a href="list.html">在线课程</a></li>
							<li><a href="list.html">人才招聘</a></li>
						</ul>
					</div>
					<div class="nav_mid_right">
						<form action="" method="post">	
							<input type="text" value=""/>
						</form>
					</div>
				</div><!--navmid结束-->
				<div class="nav_right">
				</div>
			</div><!--导航结束-->
		</div><!--wrap结束                -->

/*导航区域*/
.nav{
	height:40px;
}
.nav_left{
	width:10px;
	background: url(../img/nav_left.jpg) no-repeat;
	float: left;
	height: 40px;
}
.nav_mid{
	width: 980px;
	background: url(../img/nav_bg.jpg) repeat-x;
	float: left;
	height:40px;
}
.nav_right{
	width: 10px;
	background: url(../img/nav_right.jpg) no-repeat;
	float:left;
	height:40px;
}

这个不能对齐 是什么原因呢？