thinking to invite parents for my graduation. I am currently a permanent resident.
What are the docs. do i need to provide? I know it's a different process than while in f-1 status.

Will college write a formal invitation letter? or what really matters for them to get approved? Any one with similar experience?
Do I need to fill out i-134 (affidavit of support).
Please suggest.

I know all of these documents are not required but having them do not hurt.
College does write an invitation letter upon request and i understand it's crucial.
I understand, you must have to fill out i-134.
One affidavit of support is sufficient for father/mother
1. Affidavit of support ( I-134 )
2. university invitation letter (it's crucial if you are inviting them for graduation)
3. letter to consulate (request to grant visa..you can find sample on web)
4. relationship certificate or
5. birth certificate
6. employment verification letter(if you're employed else source of income)
7. h1b visa document/ GC Document
8. paystub
9. tax document(w2)
9. bank letter
10. stock letter (if You have any)
11. university transcript (if you have)
12. your passport, ssn
13. tax return document

You need to fill up DS 160 form(visitor visa if you're applying for visitor visa).
Separately for each candidate(Father and Mother)
I filled up the application myself and emailed them the PDF application form with the confirmation page.

Few days back I did it and they got the visa. :)

They went to Nabil bank(opposite to US Embassy in KTM), submitted the application and confirmation page of application along with fee and they got the interview very next day and got the visa too.

Good Luck.

anyone else with some more info?

Contact your International Student adviser, request an invitation letter and send it to your parents. That's it!! Your parents have to do paperwork back home which they will know upon scheduling an interview. I don't think you have to do anything here other than this. Good Luck! (Based on my prior experience)

I don't understand why 'imagine' has to show bank details and all stuffs posted above. Since imagine is calling parents in his/her graduation, parents should have sufficient documents to show that they are returning back to Nepal. All I had sent was:
1) transcript from last semester
2) graduation application
3) letter acknowledging from the department which said I was studying, graduating at that time and requested visa to my parents [my school didn't issue invitation letter because DHS has said to stop issuing such letters to international office].

I think I need to show financial documents since I am a permanent resident and to prove that I will be their sponsor . Not sure though.

ghintang is correct. For visitor's visa, there is no need of any documentation except for invitation letter and letter from college if 'reason' is graduation. No need of transcripts either.
I've personally done it for my parents and in-laws and got visa for all four of them (at different times/years) one was for graduation another was when we were expecting baby. Once I was student other time I was on H1.

One of my friend tried to 'sponsor' with 'Affidavit of support' and wrote to the embassy, and got a response from embassy explicitly stating 'no one can sponsor' for such visa and applicants are granted visa on their own capacity.

From student visas to visitor visas, there is more hype on what documents are required. Just follow what there is on embassy's website or state department's website and you are good.

That's strange. After surfing on web for many days, accumulating references from many, i have followed the similar pattern and the result is +ve.
In the response to my thread above I did mention
"I know all of these documents are not required but having them do not hurt."
It's your call.
I think, affidavit of support (I-134) is when you're covering the expenses of your parent's(ticket, medical insurance, accommodation, food and other stuff) while they are visiting to you in usa. To cover those expenses you need to have money so you need to show the bank balance.
if this make sense, go with it else again it's your call.
here is the link for required documents
http://travel.state.gov/content/visas/english/visit/visitor.html