The acceleration mechanisms by a photosensitive onium salt for the nitroxide-mediated photocontrolled/living radical polymerization (photo-NMP) were determined. The photo-NMP of methyl methacrylate was performed by irradiation at room temperature using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator and (2RS, 2’RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator. The polymerization was accelerated in the presence of (4-tertbutylphenyl)diphenylsulfonium triflate (^tBuS) to produce a polymer with a molecular weight distribution as narrow as the polymerization in its absence. (±)-Camphor-10-sulfonic acid or 2-fluoro-1-methylpyridinium p-toluenesulfonate had no effect on the polymerization speed, suggesting that ^tBuS did not serve as the photo-acid generator for the photo-NMP. It was found that the acceleration of the polymerization was based on the electron transfer from MTEMPO into ^tBuS in the excited state to temporarily generate a free radical propagating chain end and an oxoaminium salt (OAS), the one-electron oxidant of MTEMPO. This electron transfer mechanism was verified on the basis of the fact that the photo-NMP in the presence of ^tBuS was still accelerated by triphenylamine, the electron transfer inhibitor, to partly produce a polymer with an uncontrolled molecular weight. The formation of an uncontrolled molecular weight polymer indicated the generation of a free radical propagating chain end due to the deactivation of the OAS by the triphenylamine. It was deduced that ^tBuS served as the electron acceptor from MTEMPO in the excited state to temporarily produce a free radical propagating chain end along with OAS, resulting in the acceleration of the polymerization.