It was demonstrated that Fe–N martensite (α′) films were grown epitaxially on Fe(001) seeded GaAs(001) single crystal wafer by using a facing target sputtering method. X-ray diffraction pattern implies an increasing c lattice constant as the N concentration increases in the films. Partially ordered Fe16N2 films were synthesized after in situ post-annealing the as-sputtered samples with pure Fe8N phase. Multiple characterization techniques including XRD, XRR, TEM, and AES were used to determine the sample structure. The saturation magnetization of films with pure Fe8N phase measured by VSM was evaluated in the range of 2.0–2.2 T. The post annealed films show systematic and dramatic increase on the saturation magnetization, which possess an average value of 2.6 T. These observations support the existence of giant saturation magnetization in α″-Fe16N2 phase that is consistent with a recent proposed cluster-atom model and the first principles calculation [N. Ji, X. Q. Liu, and J. P. Wang, New J. Phys. 12 063032 (2010)].