The microstructure of Fe-N films with different thicknesses prepared by reactive radio-frequency magnetron sputtering has been investigated by transmission electron microscopy (TEM) and high-resolution transmission electron micrscopy (HREM) in cross-section and plan view. The film showed a small surface roughening at a nanometer scale. Grain clusters were clearly visible in the film. The electron diffraction analysis indicated that the sputtered Fe-N film was polycrystalline with a mixture of alpha-Fe and alpha"-Fe16N2 phases. A small amount of Fe2O3 was also observed. The thickness of the film and crystal size may be important factors causing distinct differences in magnetic properties of sputtered Fe-N films.