A fluffy mesoporous graphitic carbon nitride (g-CN) was synthesized and investigated, as the first metal-free polymeric semiconductor photocatalyst, for partial oxidation of methane to methanol at a mild condition (35 degrees C) in the presence of hydrogen peroxide (H2O2). The addition of g-CN photocatalyst enhanced the methanol production with H2O2 by 10 times under visible light and this production was 3-folds that when WO3 was used. Besides, while attaining a comparable saturated production of CH3OH, the visible-light-driven reactions allowed a higher utilization ratio of H2O2 compared with those under the irradiation of simulated AM1.5G sunlight. More importantly, via electron spin resonance technique, scavenger experiments and isotope tracer investigations, as well as studies on the effects of reaction condition variations, the specific reaction mechanisms were revealed under both visible light and simulated full sunlight, respectively, which should shed some new insights on photocatalytic oxidation of CH4 to value-added chemicals.