The effect of ion irradiation during deposition, in particular the effect of the formation of mixing layers at the interface between the film and the substrate by ion mixing, on the internal stress in nickel films deposited onto silicon substrates was studied. The nickel films were prepared on silicon (100) wafers by evaporation of nickel and simultaneous irradiation with inert gas ions (ion beam and vapor deposition method). The energy of inert gas ion bombardment was varied in the range 0.5-10.0 keV. The transport ratios of vapor atoms to inert gas ions, atoms/ions, to the substrates were constant at 15. The silicon substrates were kept at a low temperature by a water cooling system. The internal stress in the nickel films was determined by measuring the change in curvature of the substrates. From X-ray analysis and annealing experiments, it is considered that one of the main causes of tensile stress in nickel films prepared on silicon substrates by IVD is the formation by ion mixing of an Ni2Si layer at the interface.