Structural transformations in nickel films exposed to N + ions were investigated through the use of electron diffraction, electron microscopy, Rutherford backscattering (RBS) of He + ions, thermal desorption spectroscopy (TDS).With an increasing exposure dose, there occurred a smooth transition of the FCC structure of Ni to the HCP phase α-Ni 3 N with the lattice parameters a=0.266nm and c=0.430nm. The completion of the transition was observed once the implanted nitrogen concentration corresponding to the stoichiometric ratio of Ni 3 N was attained, this giving evidence for the chemical nature of the structural change observed.During annealing, structure variations were traced, enabling one to observe the following four nickel nitrides: (1) α-Ni 3 N with a disordered arrangement of nitrogen atoms (HCP-phase with a=0.266nm and c=0.430nm), (2) β-Ni 3 N with an ordered arrangement of nitrogen atoms (two hexagonal lattices: one with a=0.266nm, c=0.430nm, and the other with a=0.466nm and c=0.43nm), (3) Ni 4 N having a primitive cubic lattice structure with a=0.377nm, (4) Ni 8 N having the FCC structure with a=0.725nm.The results of an investigation have led to building up a phase diagram for the Ni-N system.