How to theme webform in D6?
I found the solution.

Editing the template.php file of my custom theme and more specifically the theme_form_element() function. In that function, simply move these 3 lines where I want :

if (!empty($element['#description'])) {
    $output .= ' &lt;div class="description"&gt;' . $element['#description'] . "&lt;/div&gt;\n";
}