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Direct conversion of methane to its oxygenate derivatives remains highly attractive while challenging owing to the intrinsic chemical inertness of CH₄. Photocatalysis arises as a promising green strategy which could stimulate water splitting to produce oxidative radicals for methane C−H activation and subsequent C−C coupling. However, synthesis of a photocatalyst with an appropriate capability of methane oxidation by water remains a challenge using an effective and viable approach. Herein, ceria nanoparticles with abundant oxygen vacancies prepared by calcinating commercial CeO₂ powder at high temperatures in argon are reported to capably produce ethanol and aldehyde from CH₄ photocatalytic oxidation under ambient conditions. Although high-temperature calcinations lead to lower light adsorptions and increased band gaps to some extent, deficient CeO₂ nanoparticles with oxygen vacancies and surface Ce^III species are formed, which are crucial for methane photocatalytic conversion. The ceria catalyst as-calcinated at 1100 °C had the highest oxygen vacancy concentration and Ce^III content, achieving an ethanol production rate of 11.4 µmol·g_cat^−1·h^−1 with a selectivity of 91.5%. Additional experimental results suggested that the product aldehyde was from the oxidation of ethanol during the photocatalytic conversion of CH₄.