We have synthesized Fe 1 6 N 2 films by nitrogen ion implantation into 250nm and 50-nm-thick Fe films sputter-deposited on MgO(100) substrates. In the 250-nm-thick films, Fe 1 6 N 2 and α'-martensite were formed during implantation of nitrogen ions at room temperature. The volume fractions of Fe 1 6 N 2 and α'-martensite phases estimated from integrated intensity of X-ray diffraction peaks were 12 and 39%, respectively. In the 50-nm-thick films, however, only α'-martensite was formed during implantation of nitrogen ions at room temperature and at 30K, and the volume fraction of α'-martensite exceeded 90%. Fe 1 6 N 2 was formed by post-annealing at 423K for 10h. Before annealing, the implanted films were coated with Cu films in order to prevent the release of nitrogen from the surface during the annealing. The volume fractions of Fe 1 6 N 2 and α'-martensite phases after the post-annealing were 36 and 50%, respectively. The effects of film thickness or implantation temperature on nitride or defect formation were examined by X-ray diffraction studies. A saturation magnetization of a 250-nm-thick Fe film implanted with nitrogen ions was larger than that of an unimplanted Fe film by about 5%.