How can exclude some directory for mod_pagespeed using htaccess rules....
note : I am required to disable only some directory not all files and folders
You can simply set ModPagespeed off in an .htaccess file. This will disable all rewiting of HTML in that directory. It will not stop mod_pagespeed from rewriting images/CSS/JS in that directory.
To disable rewriting of all HTML and resources, you can add ModPagespeedDisallow http://www.example.com/subdir/* anywhere in your config. See documentation for more info. Note that this requires you to specify the entire URL, not just a relative path.