The γ iron nitride (nitrogen austenite) was subjected to high magnetic field process in order to drive the fcc→bct martensitic transformation. Molar fraction of martensite monotonically increased with increasing the magnetic field and reached 94% at 35T. With a combination of magnetization and Fe57 Mössbauer spectroscopy data, magnetization of bulk processed α′ phase with 9.6at.% N is determined to be 229emu∕g, the same as that for dc sputtered thin films.