GET serach/tweets.json works fine but GET friends/list.json returns 401 unauthorized


Need Help :
I have used oath_token and oauth_token_secret from the application detail page and building the Authorization header.

I am not able to sort out that why one(GET serach/tweets.json) API is working fine and second one (GET friends/list.json) is not??

Is there any special requirements for GET friends/list.json which are not in documentation?