I am a little confused about the citizenship for my niece.

She's 2 months baby, born in Bangkok. Her father is Thai and Her mother is Japanese.

I wonder which national passport is necessary for her, Thai or Japanese?

The mother said she needs both Thai and Japanese passport.

If so, what would be important document to apply and where would she could apply for the passport?

Your prompt reply be greatly appreciated.

Khob Khun Ka!

Both passports should be obtained. But before age 22 she must make a choice or loss of Japanese citizenship will result it seems by current law.

Birth certificates are most important item. She should check with Embassy of Japan. As born here of a Thai father they can visit MFA passport office to obtain Thai passport.

What needs to be done also depends on the fact if the parents are married to eachother or not. If marreid.