<script type="text/javascript">

function isNumberKey(evt) {

var charCode = (evt.which) ? evt.which : event.keyCode;

if (charCode >= 48 && charCode <= 57) {

return true;

}

else {

alert('number only');

return false;

}

}

</script>

<asp:TextBox ID="TextBox2" runat="server"

onkeypress="return isNumberKey(event)" ></asp:TextBox>