A study has been made of the relationship between crystal structure and hardness of N-implanted Fe-surface layers. The substrates used were polycrystalline Fe-sheets 1 mm thick with a purity of 99.9 %. The ion implantation of 14N+ was performed with doses ranging from 7.5×1016 to 1×1018 ions/cm2 at an energy of 100 keV. The substrate temperatures during implantation were kept at -40, 20, 100 and 200 °C. The crystal structure of N-implanted layers was identified by X-ray diffraction method (XRD). The near surface hardness was measured by a Knoop hardness tester. The XRD patterns revealed that the iron-nitride peculiar to each implantation condition was formed, and the iron-oxide was embedded in the nitrides in case of higher temperature implantation. N-implantation led to hardness increase, whose degree also depended on implantation conditions. The comparison between the structure and hardness of implanted layers indicated that, among the phases of iron-nitrides produced by 20 °C implantation, ε-phase is the hardest and the γ-Fe203 and γ'-Fe4N formed by 200 °C implantation are harder than the ε-phase nitrides. These results are discussed with respect to the surface composition estimated by Rutherford backscattering spectrometry (RBS) using 1.5 MeV 4He+-ions.