Solution:
if your menu structure is like below:
<ul class="parent-menu">
<li><a href="#">Menu1</a></li>
<li><a href="#">Menu1</a>
<ul class="sub-menu">
<li><a href="#">Sub Menu1</a></li>
<li><a href="#">Sub Menu1</a></li>
<li><a href="#">Sub Menu1</a></li>
</ul>
</li>
<li><a href="#">Menu1</a></li>
</ul>
Then you can use css like this:
ul.parent-menu li {
position: relative;
}
ul.parent-menu li ul.sub-menu {
display: none;
position: absolute;
left: 0;
top: 100%;
}
ul.parent-menu li:hover > ul.sub-menu {
display: block;
}