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.