The Cu-Mn mixed oxide catalysts with different molar ratios of Cu/(Cu+Mn) prepared by co-precipitation method have been investigated in CO oxidation at 30℃. The catalysts used in this study were characterized by X-ray Diffraction (XRD), N2 sorption, X-ray photoelectron spectroscopy (XPS), and H2-temperature programmed reduction (H2-TPR) to correlate with catalytic activities in CO oxidation. The N2 adsorption-desorption isotherms of Cu-Mn mixed oxide catalysts showed a type 4 having pore range of 7-20 nm and BET surface area was increased from 17 to 205 m2·g^(-1) with increasing of Mn content. The XPS analysis showed the surface oxidation state of Cu and Mn represented Cu^(2+) and the mixture of Mn^(3+) and Mn^(4+), respectively. Among the catalysts studied here, Cu/(Cu+Mn) = 0.5 catalyst showed the highest activity at 30℃ in CO oxidation and the catalytic activity showed a typical volcano-shape curve with respect to Cu/(Cu+Mn) molar ratios. The water vapor showed a prohibiting effect on the efficiency of the catalyst which is due to the competitive adsorption of carbon monoxide on the active sites of catalyst surface and finally the formation of hydroxyl group with active metals.