ZnO/reduced graphene oxide (RGO) nanocomposite with many zinc and oxygen vacancies, synthesized in a simple solvothermal reaction exhibits significant photocatalytic activity. Photoluminescence and electron paramagnetic resonance measurements indicate that the zinc vacancies and oxygen vacancies were generated on the ZnO surface, and were crucial to that photocatalytic behavior. The photodegradation of methylene orange was significantly reduced by the addition of h+ and OH scavengers. Both zinc and oxygen vacancies cause effective charge separation in the photodegradation of methylene orange, which markedly inhibits the recombination of charges. The advanced photocatalytic behavior of the ZnO/RGO composite is discussed in detail herein.