What's problem with .htaccess?

[eluser]vps4[/eluser]
the .htaccess file in site's root

Code:

RewriteEngine ON

RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule . /index.php [L]

I have a sub-directory "admin"
I put an other .htaccess file to protect admin panel

Code:

AuthType Basic
AuthName "Please Login"
AuthUserFile "/home/xxx/.htpasswds/public_html/aaa.com/admin/passwd"
require valid-user

when I visite http://aaa.com/admin/

the second .htaccess will NOT work. It redirect to http://aaa.com/

If remove the .htaccess file in the site root, admin directory .htaccess will work fine.

How can I do?