Abstract: Co₃O₄/CeO₂ catalysts were prepared by co-precipitation-oxidation method and characterized by using X-ray diffraction (XRD), surface area (S_BET) measurement and temperature-programmed reduction (TPR). The effect of cobalt content in Co₃O₄/CeO₂ on its physical properties and catalytic performance in CO oxidation was investigated under both the humid and dry conditions. The results indicated that the Co₃O₄/CeO₂ composite with a Co/Ce atomic ratio of 9 (calcined at 723 K) exhibits much smaller particle size and higher surface area than pure Co₃O₄; the addition of cerium to Co₃O₄ is beneficial to obtain Co₃O₄ with small particle size. The average particle size and BET surface area of Co₃O₄ in the Co₃O₄/CeO₂ composite (Co/Ce atomic ratio=9) calcined at 538 K are 7.2 nm and 167.6 m²/g, respectively. Accordingly, the Co₃O₄/CeO₂ composite exhibits excellent redox property.