Abstract: A series of manganese-zirconium composite oxides were prepared by citric acid complexing method and characterized by XRD, H₂-TPR, XPS and SEM; their performance in the catalytic reduction of NO by CO was investigated. The results show that Mn₃O₄ is the main phase for MnO_x in the Mn-Zr composite oxide; an increase in the Zr content can promote the dispersion of Mn₃O₄ and reduce the average grain size of Mn₃O₄. Mn may exist in the form of Mn²⁺, Mn³⁺ and Mn⁴⁺ ions; the content of (Mn³⁺ + Mn⁴⁺) and the quantity of surface adsorbed oxygen (OA) increase after the addition of Cu and Ce, which is beneficial to enhancing the catalytic activity. The pristine Mn-Zr-O composite shows a relatively low activity in the catalytic reduction of NO by CO; after adding Cu, the Mn-Cu-Zr-O composite exhibits much higher activity than Mn-Zr-O; moreover, the activity of Mn-Cu-Ce-Zr-O composite is even enhanced by adding Ce. For the catalytic reduction of NO by CO over the Mn-Cu-Ce-Zr-O composite at 350℃ and with a space velocity of 18000 h^-1, the CO conversion and NO conversion are 89.17% and 91.70%, respectively.