problem of remove index.php from uri

[eluser]vantao[/eluser]
my project structure is:
----root-
-system-
-application
-cache
-...
-index.php
my .htaccess is:
RewriteEngine on
RewriteCond $1 !^(index\.php|images|robots\.txt)
RewriteRule ^(.*)$ /index.php/$1 [L]

while i put .htaccess with the same level with system folder and index.php,it returned nothing.while i check the access.log,there is a 404 message. then i put .htaccess under system folder,the same level with application folder, it works,but only to index.php,while i'm trying to access other target without typing index.php in uri, i got a 404 message too.

what's wrong?anyone help me?


thanks!