Ag2O/g-C3N4 composites synthesized in this study were applied in the photocatalytic degradation of phenol under UV- and visible-light irradiation. X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy analysis demonstrated that Ag2O nanoparticles were well distributed on the surface of g-C3N4, and the heterostructure of Ag2O/g-C3N4 was formed. Compared with pure g-C3N4 and Ag2O, the Ag2O/g-C3N4 composite (8:1) displayed much higher photocatalytic activities in phenol degradation under UV- and visible-light irradiation. The degradation rate constant of 8:1 was 0.069 min1 under visible light, which was almost 230 and 2.1 times more than that of pure g-C3N4 and Ag2O, respectively. Moreover, the formation of a certain amount of Ag-0 on the surface of Ag2O under illumination contributed to the high stability of Ag2O/g-C3N4 photocatalysts. It was also found that photogenerated holes during the photocatalytic process played the predominant role in phenol degradation. The improved photochemical reactivities were attributed to the formation of the heterostructure between Ag2O and g-C3N4, the strong visible-light absorption, and the high separation efficiency of photoinduced electronhole pairs resulting from the highly dispersed Ag2O particles.