In this study, catalysts with different loadings of CuO supported on TZ20 (TiO2:ZrO2 = 80:20) composite oxides were prepared by the co-precipitation method and characterized by XRD, TEM, and H-2-TPR measurements, and N2O decomposition experiments were conducted to determine the number of Cu sites. In addition, the relationship between the CuO states and CO oxidation activities of CuO/TZ20 was revealed. In our previous study, the CO oxidation activities of the CuO catalysts were as high as those observed for precious metal catalysts after simulated diesel exhaust aging tests. The TZ20 surface is covered with three types of Cu species. These species change with increasing CuO loading, going from isolated Cu2+, CuO layer, and finally to CuO particles at high CuO loadings. The CuO layer was the most readily reduced, as well as the most active site for CO oxidation. The amount of Cu forming CuO layers increased with increasing CuO loading; however, at CuO loadings larger than 5 wt%, the amount of Cu forming CuO layers and the CO oxidation activity both remained constant. Additionally we indicated that adjusting the calcination condition caused the redispersion of CuO particles to the CuO layer and had the potential to improve the CO oxidation activities of CuO/TZ20.