You need to include anchor link to Drupal menu (<a name="anchor"></a>).
If you enter “node/15#anchor” to the “Path” field, Drupal will convert “#” to “%2523″ and you will get this on the output: “/node/15%2523anchor“. In this article I would tell you how to fix this problem.

Note:
Drupal shows ‘strict xhtml 1.1’ and anchors like this one  <a name="anchor"></a> don’t work.

We will replace this tag with ‘id’ tag in xhtml 1.1. This tag could be used for any tag.
For example, <p id="anchor"></p> and you can refer to an anchor as usual: example.com#anchor

Options

• Show full URL
• Redefine output function to show links in menu

Show full URL

Anchors will work in menu links if you specify full URL (http://www.example.com/example#anchor instead of /example#anchor). But full path will make your site less extensible. So you can’t always use this solution.

Redefine output function to show links in menu

You need to redefine the function – add the following code to template.php of your theme:

function phptemplate_menu_item_link($item, $link_item) {
// Convert anchors in path to proper fragment
$path = explode(‘#’, $link_item[‘path’], 2);
$fragment = !empty($path[1]) ? $path[1] : NULL;
$path = $path[0];
return l(
$item[‘title’],
$path,
!empty($item[‘description’]) ? array(‘title’ => $item[‘description’]) : array(),
!empty($item[‘query’]) ? $item[‘query’] : NULL,
$fragment,
FALSE,
FALSE
);
}