Abstract: Composite oxides of copper and manganese are widely used in oxidation reactions, and the main active component is copper-manganese spinel, whereas the copper oxide and manganese oxide show extremely low activity, despite that they are beneficial for CO oxidation. In this paper, it was found that the synergistic effect of Cu_1.5Mn_1.5O₄ and CuO could promote the catalytic oxidation of CO. The catalysts were prepared using the citric acid complexation method and they were characterized through combined the techniques of N₂-adsorption desorption, XRD, H₂-TPR, TEM, CO-TPD and O₂-TPD. The catalytic performance of various catalysts in CO oxidation was then evaluated. The results confirmed that Cu_1.5Mn_1.5O₄ modified with CuO exhibited the best catalytic performance and the highest unit surface activity (defined as the CO conversion rate per unit surface area of catalyst). CuO and Cu_1.5Mn_1.5O₄ had a synergistic effect, where O₂ was activated by CuO and it then interacted with CO, activated by Cu_1.5Mn_1.5O₄, to form CO₂, thus increasing the catalytic activity.