<table class="table table-bordered">
  <th colspan="3">Outer Table</th>
  <tr>
    <td>This is row one,column 1</td>
    <td>This is row one,column 2</td>
    <td>
     <table class="table table-bordered">
       <th colspan="3">Inner Table</th>
       <tr>
         <td>This is row one,column 1</td>
         <td>This is row one,column 2</td>
         <td>This is row one,column 3</td>
       </tr>
     </table>
    </td>
  </tr>
</table>

演示：http://jsbin.com/qilebevo/1/