Reactive diffusion of the Ni/Si system has been studied by annealing nickel thin film on (100) silicon crystal. The measurement of the NiSi sheet resistance as a function of the annealing temperature and the type of annealing (Rapid Thermal Annealing and spike one) has been investigated. A kinetic model based on multiphase diffusion has been developed that fits experimental sheet resistance data. Residual stress in the thin film, measured by a curvature measurement technique, is correlated with the nature of the phases in the film. Finally the viscoplastic mechanical behavior of the Ni2Si and NiSi phases is analyzed in the case of low and fast thermal ramps.