Hi
I have presently found one offer job form brown university in Rhode Island in USA. I will be post-doc researcher form two years. My sponsor will apply J1 visa. I am Iranian and I live and work as researcher in Japan. However, my passport is Iranian. Do I need to pay tax. I heard from my sponsor, there is one rule, that some nationalities will not pay tax under J1 type visa. Or they will claim for exemption of payment. Is it including my situation?

I doubt if Iran has tax treaty with U.S. In the absence of tax treaty, you do not get any benefit. Read IRS publication 901. Tax Treaties.

Agreed. Unless you became a legal resident of Japan, then Iran is considered your country of legal residence, and there is NO tax treaty with Iran, so no income tax exemption.