The best way to do this is strictly separate your table into two different tables - header and body:
<div class="header">
<table><tr><!-- th here --></tr></table>
</div>
<div class="body">
<table><tr><!-- td here --></tr></table>
</div>
.body {
height: 100px;
overflow: auto
}
If your table has a big width (more than screen width), then you have to add scroll events for horizontal scrolling header and body synchroniously.
You should never touch table tags (table, tbody, thead, tfoot, tr) with CSS properties display and overflow. Dealing with DIV wrappers is much more preferable.