I created a new drupal 7 theme and trying to implement hook_theme at template.php like this:

function mytheme_theme($existing, $type, $theme, $path){
    return array(
        'mytheme_header'=>array(
            'template'=>'header',
            'path'=>$path.'/templates',
            'type'=>'theme',
        ),
    );
}

then I placed header.tpl.php into templates directory and cleared all caches, and call theme function:

theme('mytheme_header', $vars);

but it did not work and render page with header.tpl.php. What's wrong with my code?