The mechanism of reverse water gas shift reaction over Cu catalyst was studied by pulse reaction with QMS monitoring, temperature programmed desorption (TPD) and temperature programmed reduction (TPR) of Cu/SiO2 catalyst. The reduced and/or oxidized copper offered low catalytic activity for the dissociation of CO2 to CO in the pulse reaction study with 1 ml volume of He/CO2, but the rate of CO formation was significantly enhanced with H-2 participating in the reaction. The TPD spectra of CO2 obtained by feeding H-2/CO2 over copper at 773 K provided strong evidence of the formation of formate at high temperature. The formate derived from the association of H-2 and CO2 is proposed to be the key intermediate for CO production. The formate dissociation mechanism is the major reaction route for CO production.