Abstract: Clean and Oxygen-modified Cu（100） surfaces have been used to model the metallic and the partially oxidized copper surfaces respectively.Activation energies for elementary reactions in volved in the water-gas shift（WGS）reaction over Cu（100）and Cu（100）-p（2×2）O surface have been calculated using bond order conservation-Morse potential approach.Judging by the activation energies,we concluded（i）WGS reaction proceeds via both surface redox and formate mechanisms;（ii）COs is mainly oxidized to CO₂s by OHs rather than by Os;（iii）the dissociative adsorption of water is expected to be the rate-determining step;（iv）WGS reaction is more favorable over the metallic copper surface than that over the partially oxidized copper surface.