A series of CeO2-Co3O4 mixed oxide catalysts with different Co/(Co+Ce) atomic ratios were synthesized by citric acid sol-gel method and used for catalytic oxidation of formaldehyde (HCHO). Many techniques such as Brumauer-Emmett-Teller (BET), X-ray diffraction (XRD), scanning electron microscopy (FE-SEM), temperature programmed reduction (H2-TPR), temperature-programmed desorption (O2-TPD) and X-ray photoelectron spectroscopy (XPS) were used to characterize catalysts. The results of catalytic performance tests showed that the catalyst CeO2-Co3O4 with Co/(Co+Ce) ratio of 0.95 exhibited the best performance, and the temperature of complete oxidation of HCHO was 80 °C. The analytical results indicated that the addition of CeO2 enhanced the specific surface area of Co3O4 and the fine dispersion of both of them. Moreover, the strong interaction between CeO2 and Co3O4 resulted in the unique redox properties, which enhanced the available surface active oxygen and led to high valence state of cobalt oxide species. All those effects played crucial roles in the excellent performance of CeO2-Co3O4 for the HCHO oxidation.