Hi Eklo Yatri,
These are the documents my parents gathered before interview.
-Invitation letter from my University and my personal letter to consular requesting visa (do not forget to mention little bit about you, what you do and where will your parents spend time in US).
-My I-20, Visa and recent transcript.
-A relationship certificate from Municipality or VDC (should be translated and do not forget to include all the siblings even if they are in Nepal. Including all sibling shows that your parents have other kids in Nepal and they will definitely return for the sake of other kids)
-Letter from your parent's employer. If you parents are working it would be nice to have a letter from the employer talking about them, nature of their job, how long they are working and something about leave.
-A letter from your parent's boss will also help (mention that your parents will continue their job after they return from the trip).
-Property evaluation (house, land, business or services) and bank balance (amount that your parents could spend during their visit or just to show that your parents have property there in Nepal which will make them to come back)
This should be enough.

Good luck,