Nitrogen ions were implanted into [100] oriented alpha-Fe thin films on MgO (100) substrates at room temperature. The films were annealed at a low temperature of 473 K. alpha’-Fe16N2 and alpha’-martensite phases were formed and the volume fractions of these nitride phases were estimated from the X-ray diffraction patterns. When the film thickness was 250 nm, alpha"-Fe16N2 was formed directly by ion implantation and the maximum volume fraction was about 12%. For the case of 50 nm thick films, no alpha"-Fe16N2 but alpha’-martensite was formed after nitrogen-ion implantation, a nd the volume fraction of the martensite exceeded 90%. By post annealing at 473 K, alpha"-Fe16N2 was formed, when the implanted specimens were coated with gold or copper films. The volume fraction of alpha"-Fe16N2 reached about 36%. A SQUID magnetic measurement showed that the saturation magnetization of the nitrogen-implanted 250 nm thick iron films was a few per cent larger than that of unimplanted iron films.