Diffusion of Ni atoms in P-doped Czochralski-grown (Cz) monocrystalline Si has been studied using the Ni63 radiotracer and autoradiographic and sectioning techniques. The diffusivity of Ni in P-doped Cz Si is described by D=6×10-4 exp[(-0.76 eV)/kT] cm2/sec over the temperature range of 220 540°C. The solubility of Ni in Si at 800°C is about 1016 atoms/cm3. At low temperatures, the solubility is uncertain due to the limit of tracer detection. A critical examination has been made on the published values of Ni diffusion in Si which show a scatter of 10 orders of magnitude among the various investigations. The investigations which report low-diffusion coefficients and large activation energies are suspect due to the problems encountered with the surface conditions and limitations imposed on Ni detection by low solubility and fast diffusion. Our data on Ni diffusion in Si substantiate an interstitial mechanism without intervention of the native defects in Si.