In this work, the significantly enhanced photocatalytic performances and higher reaction rate of the hierarchical ZnO microspheres – initially prepared by a solvothermal method without surfactants or templates and decorated with Ag nanoparticles by a photoreduction method – were found for the degradation of methylene blue (MB) under both ultraviolet (UV) and visible light irradiation. Various characterization results confirmed that the modification of silver with an optimal amount can effectively extend the absorption spectrum to the visible region and inhibit a recombination of photo-induced charge carriers. Moreover, the reason for promoted photostability and the possible mechanism for the enhanced photocatalytic activity of the as-prepared Ag/ZnO composites under UV or visible light irradiation were also systematically investigated and discussed.