SOURCE

function insRow()
  {
  var x=document.getElementById('myTable').insertRow(0)
  var y=x.insertCell(0)
  var z=x.insertCell(1)
  y.innerHTML="NEW CELL1"
  z.innerHTML="NEW CELL2"
  }

<table id="myTable" border="1"> 
  <tbody>
   <tr> 
    <td>Row1 cell1</td> 
    <td>Row1 cell2</td> 
   </tr> 
   <tr> 
    <td>Row2 cell1</td> 
    <td>Row2 cell2</td> 
   </tr> 
   <tr> 
    <td>Row3 cell1</td> 
    <td>Row3 cell2</td> 
   </tr> 
  </tbody>
 </table> 
 <br> 
 <input type="button" onclick="insRow()" value="插入行">