How form layout of user login page can be changed Drupal 7 without altering its tpl file ? I want to change the layout of user login form in Drupal 7. I don't want to change or create its tpl file for that purpose. Is there any module which will works for me ?
Have a look at hook_form_alter. You have to change the #weight property of the form elements.

If you don't know what I'm talking about have a look at the FAPI reference and quick start guide.