Photocatalytic oxidation of methane to value-added chemicals is a promising process under mild conditions, nevertheless confronting great challenges in efficiently activating C–H bonds and inhibiting over-oxidation. Herein, we propose a comprehensive strategy for the selective generation of reactive oxygen species (ROS) by regulating the sizes and facets of Au nanoparticles loaded on ZnO. For photocatalytic methane oxidation at ambient temperature, a high oxygenates yield of 36.4 mmol·g⁻¹·h⁻¹ with a nearly 100% selectivity has been achieved over the optimized 1.0% Au/ZnO-9.6 (1% Au with (111) facet and 9.6 nm size on ZnO) photocatalyst, exceeding most reported literatures. Mechanism investigations reveal that 1.0% Au/ZnO-9.6 with the medium size and Au (111) facet guarantees the favourable formation of superoxide radicals (·OOH) through mild oxygen reduction, ultimately leading to excellent photocatalytic methane oxidation performance. This work provides some guidance for the delicate design of photocatalysts for efficient photocatalytic methane oxidation and oxygen utilization.