Nanosized CeO2 and supported Au catalysts were synthesized and their catalytic performance for CO preferential oxidation in H-2-rich streams was evaluated. Transmission electron microscope (TEM), N-2 isothermal adsorption/desorption, X-ray diffraction (XRD), temperature programmed reduction (H-2-TPR), X-ray photoelectron spectroscopy (XPS) and in-situ DRIFTS techniques were adopted to characterize their structure property, redox property as well as dispersion and valence states of the active Au species. The results demonstrate the strong interaction between highly dispersed gold species (grain size < 2 nm) and CeO2 in catalysts with low gold loading, which promotes the reduction of surface and sub-surface oxygen and the stabilization of high-valent Au3+ species. Meanwhile, it creates more lattice defects and benefits to the enhanced mobility of lattice oxygen and the activation of gas oxygen, thus contributing to the low-temperature CO preferential oxidation. However, over high Au content (>= 0.6%) would lead to the agglomeration of gold nanoparticles and the formation of Au-0 species, which weakens the gold-ceria interaction and decreases the active sites. On the other hand, the formation of Au-0 species with large grain size would accelerate the dissociation of H-2, which is the main reason for the decreasing of O-2 selectivity toward CO over Au/CeO2 catalysts.