Finite amounts of noble metals have been widely introduced as surface plasmon resonance (SPR) mediators and reductive cocatalysts for solar-driven energy conversion. At present, knowledge of the roles of metal loading is multifarious and may be one-sided in some cases. In addition, the catalytic roles which metals play in photocatalytic oxidation have been rarely discussed. It is necessary to explore the equilibrium between plasmon resonance and surface catalysis over metallic nanostructures. Herein, Au NPs with various loading amounts (0.25?1 wt %) and particle sizes (3?20 nm) were attached to CeO2 by photodeposition. Aerobic oxidations of propylene under simulated sunlight and visible (>420 nm) light irradiation were selected as probe reactions. Both processes exhibited similar humplike activity dependence upon Au NP addition, with a peak at 0.67 wt % loading and a size of 8.4 nm. Modifications to the whole photocatalytic process brought by metal attachment have been integrally examined, concerning both the photoexcitation and surface catalysis steps. With an increase of Au loading, the induced SPR photoabsorption, charge separation, and resonant energy transfer were enhanced, whereas outgrown Au NPs (>10 nm) led to the saturation of exposed active sites for reactant adsorption as well as distinct passivity to O2 dissociation. Therefore, photoexcitation and surface catalysis present opposite dependence on Au NP size and codetermine the final photocatalytic performance in propylene oxidation. An integral consideration of the above two aspects should be instructive for a better understanding of SPR-enhanced photocatalysis and the design of efficient metal?semiconductor systems for ideal solar energy conversion.