<div class="menu-main-menu-container">
   <ul id="menu-main-menu" class="menu">
   <li id="menu-item-28" class="menu-item menu-item-type-post_type current-menu-item page_item page-item-21 current_page_item menu-item-28"><a href="http://www.bemang.com/">Trang nhà</a></li>
    <li id="menu-item-29" class="menu-item menu-item-type-post_type menu-item-29"><a href="http://www.bemang.com/blog/">Blog</a></li>
    <li id="menu-item-30" class="menu-item menu-item-type-post_type menu-item-30"><a href="http://www.bemang.com/gioi-thieu/">Giới thiệu</a></li>
    </ul>
   </div>

<li id="menu-item-28" class="menu-item menu-item-type-post_type current-menu-item page_item page-item-21 current_page_item menu-item-28"><a href="http://www.bemang.com/">Trang nhà</a></li>
   <li id="menu-item-29" class="menu-item menu-item-type-post_type menu-item-29"><a href="http://www.bemang.com/blog/">Blog</a></li>
   <li id="menu-item-30" class="menu-item menu-item-type-post_type menu-item-30"><a href="http://www.bemang.com/gioi-thieu/">Giới thiệu</a></li>

'container'      => '',
'items_wrap'    => '%3$s',

add_filter('wp_nav_menu_args', 'prefix_nav_menu_args');
function prefix_nav_menu_args($args = ''){
    $args['container'] = false;
    return $args;
}