i am doing btech in mechanical engg.i m in final year. .after btech should i go for job in an automobile company or should i persue post graduate diploma in thermal power enng. from npti nangal

The course you mention is a specialised course and will be an added advantage for you to get into firms like NTPC.It does not guarantee job else where in other industries. But if you are having offers now, then do not rethink now.