AuNP-modified CeO2 nanomaterials with three different morphologies were prepared and their antioxidant abilities were evaluated by the UV method; then, the mechanism of the effect of AuNPs was further explored. It was found that AuNPs' effect on the antioxidant properties of CeO2 varied with different morphologies and sample concentrations. At lower concentrations, AuNPs improved the antioxidant abilities of CeO2 nanorods and nanoparticles and reduced that of CeO2 nanocubes. However, at higher concentrations, the antioxidant activities of Au/CeO2 nanorods and nanoparticles were decreased and that of Au/CeO2 nanocubes was enhanced. It was observed that AuNPs promoted the generation of CO3+ on the surfaces of CeO2 nanorods and nanoparticles, causing increased antioxidant activity, while they inhibited the generation of CO3+ on nanocubes, accompanied with decreased activity. In addition, at higher concentrations, more exposed active sites on the surfaces of Au/CeO2 nanorods and nanoparticles were hidden due to severe aggregation, resulting in decreased antioxidant activity compared to CeO2; less aggregation was found for Au/CeO2 nanocubes, and their antioxidant activity was better than that of CeO2. Furthermore, the behaviour of nanocubes was found to be opposite to that of nanorods and nanoparticles, possibly attributed to the particularity of (1 0 0) crystal planes, which may change the way that electrons transfer at the interface.