The in-diffusion and the annealing processes of nickel in silicon are studied experimentally at 900°C to distinguish between vacancies and self-interstitials as the dominant point defects in silicon. The experimental results show that, in both the in-diffusion and the annealing processes, the concentration of substitutional nickel atoms varies with time t, following the linear dependence on t during a short time after the onset of both processes, and that the time constant of the annealing process is indpendent of the initial concentration of nickel.These results support the theory that nickel in silicon diffuses by the dissociative mechanism and the dominant point defects in silicon are vacancies.