In traditional models of P diffusion in Si, vacancy assisted diffusion mechanism has been assumed. More recently, experiments have determined that for intrinsic P diffusion in Si, the interstitial assisted diffusion mechanism dominates. We have performed ab-initio pseudopotential calculations to study P diffusion in Si. Special care is taken with regard to structure minimization, charge state effects and corrections. We calculated the defect formation energies and migration barriers for the various competing P-interstitial complex diffusion mechanisms for low concentration P region, as well as the energetics of different charge states P-vacancy complex diffusion. For interstitial mediated diffusion of P-Sii pair in Si, we find the overall diffusion activation energies calculated are 3.1 eV for neutral case, and 3.4 eV for +1 charge case. This is in agreement with experimental observation that the interstitial mechanism dominates for intrinsic P diffusion in Si. For vacancy mediated diffusion, our calculations are in agreement with previous calculations result in the neutral case. We obtained the lower bounds for diffusion activation energy of 3.8 eV for (PV)0 and 3.4 eV for (PV). A further evaluation of the numbers would require a proper treatment of the energy states in the band-gap due to Jahn-Teller relaxations.