Elastic band method has been used to calculate the activation energy for Li diffusion in the olivine structured LiFePO4 and FePO4. Screening from anions, the valence of the nearest neighbor transition metal, lattice parameters, are identified to be important factors to determine the activation energy for Li diffusion. The calculated activation energy in LiFePO4 and FePO4 are 0.5 eV and 0.27 eV, respectively. It is dependent on the concentration and configuration of lithium ions in the phase boundary regions and a kinetic model for Li extraction and insertion process is proposed. (C) 2010 Elsevier B.V. All rights reserved.