Copper catalysts supported on magnesia promoted ceria are synthesized and used for NO reduction by CO. The supports and the subsequent Cu catalysts are prepared by citric acid sol-gel and impregnation methods, respectively. The results of N-2-physisorption, XRD, EPR and TPR measurements indicate that the Mg addition promotes not only the dispersion of CuO, but also the formation of Cu-O-Ce solid solution. The highest amount of Cu-O-Ce solid solution is found in catalyst modified with 5 wt.% MgO. Based on the catalytic performance, two active sites for the reduction NO by CO are proposed: copper matrix and Cu-O-Ce solid solution. The former does not require the involvement of oxygen vacancy and is more active at low temperature range (T < 200 A degrees C), while the latter shows higher NO conversion at temperature above 200 A degrees C, due to the easy regeneration of oxygen vacancy. Also, it is found that the NO conversion over copper matrix is suppressed, while that over Cu-O-Ce solid solution is stimulated in high CO concentration.