By oxidation of silver nanoparticles (AgNPs) in two ways: thermal oxidation (TO) in molecular oxygen and cool oxidation in oxygen plasma (i.e., oxygen plasma irradiation, OPI), the efficiency and mechanism of visible light-induced selective transformation of 4-aminothiophenol (PATP) to 4,4'-dimercaptoazobenzene (DMAB) on the surface of AgNPs was explored. On the basis of the evolution of surface-enhanced Raman scattering (SERS) spectrum of PATP (10(-5) M in ethanol) with the oxidation time, it can be concluded that OPI could improve the selective transformation efficiency (eta) effectively, by 87 times for only 2 s; whereas TO could improve eta conditionally, increasing at first and then decreasing gradually to zero. The results imply that silver oxide is not the root cause of the increased eta. Combined with the results of SERS of oxygen species on the surface of AgNPs processed by the abovementioned two ways, superoxide (O-2(-)) and electrophilic oxygen atoms (O-) are suggested to be responsible for this selective transformation. Our study deepens the understanding of the mechanism of plasmonic photocatalysis and the role of silver oxide in selection transformation of organic molecules.