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Engine technologies using efficient combustion and down-sizing turbo have become important in order to reduce automotive CO2 emissions. However, the exhaust gas temperature also becomes lower by these technologies. As a result, the catalyst performance becomes lower. Therefore it is necessary to develop low temperature active catalysts to reduce emissions. This research was focused on Pd/CeO2, and it’s able to oxidize CO at low temperatures. In order to increase the catalyst activity, the addition of some elements to the CeO2 was studied. Zn addition was found to have an advantage to reduce the CO light off temperature by 60 °C. Then, we tried to clarify the cause of improvement. As a result, it made clear that the Zn addition promotes the active oxygen release from the CeO2 surface. However, repeated engine exhaust gas tests indicated a decline in purification performance.