I am getting data about certain pages from https://graph.facebook.com/pages
but i want details about posts on certain Facebook page.
how can i do this? without use of access token.
If you read the doc carefully, you'll find that you can not fetch the posts without using an access token.
But, I think you might be interested in a small work around- create an app on facebook, save the app access token (app_id|app_secret) and use it to get the public posts of the page.