Two types of CeO2 nanocubes (average size of 5 and 20 nm respectively) prepared via the hydrothermal process were selected to load gold species via a deposition‐precipitation (DP) method. Various measurements, including X‐ray diffraction (XRD), Raman spectra, high resolution transmission electron microscopy (HRTEM), in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and temperature‐programmed reduction by hydrogen (H2‐TPR), were applied to characterize the catalysts. It is found that the sample with ceria size of 20 nm (Au/CeO2‐20) was covered by well dispersed both Au3+ and Auδ+ (0<δ<1). For the other sample with ceria size of 5 nm (Au/CeO2‐5), Au3+ is the dominant gold species. Au/CeO2‐20 performed better catalytic activity for CO oxidation because of the strong CO adsorption of Auδ+ in the catalysts. The catalytic activity of Au/CeO2‐5 was improved due to the transformation of Au3+ to Auδ+. Based on the CO oxidation and in situ DRIFTS results, Auδ+ is likely to play more important role in catalyzing CO oxidation reaction.