• CO 2 RR reaction mechanism on cu and CuO was studied by DFT. • The favorable pathways for CO 2 RR into CH 3 OH production were determined. • Surface h on CuO has significant effect on CO hydrogenation reaction kinetics. • New fundamental insights on CO 2 RR mechanism were unveiled. Cu oxide and Cu are widely used catalyst in CO 2 reduction reaction (CO 2 RR) for methanol (CH 3 OH) production. However, due to the complexity of the oxidation states on catalyst surfaces, the active sites of Cu-based catalysts for CO 2 RR are still unclear. In this study, density functional theory (DFT) calculations were performed to investigate the active sites for CO 2 RR reduction into CH 3 OH on Cu (111) and CuO (111) surface. The most favored adsorption sites and reaction energetics involved in CO 2 activation and sequent CO hydrogenation steps for CH 3 OH formation were determined. It has been found that, for CO 2 reduction, the CO 2 →COOH*→CO* pathway prefers to occur on both Cu (111) and CuO (111) surfaces. More importantly, the analysis reveals that, compared with Cu (111) and clean CuO (111), sequent CO hydrogenation into CH 3 OH is promoted by the H pre-covered CuO (111), on which the surface H is set on Cu site (H Cu ). That is, in CO 2 RR, the catalytic active sites for CH 3 OH production are the H-covered Cu sites on CuO (111) surface. These new insights on the catalytic active sites for CH 3 OH production on Cu-based catalyst would potentially guide the design of efficient CO 2 RR catalysts.