ZnO was prepared using zinc acetate dehydrate (Zn(CH3COO)2·2H2O) and lithium hydroxide monohydrate (LiOH·H2O) as raw material through a microwave-assisted sol-gel method. The prepared ZnO presents the wurtzite structure. The influence of different reaction temperature and heating time to the size of ZnO was researched. The photocatalytic activity of ZnO was evaluated by the degradation rate of C.I. Reactive Blue 4 in aqueous solution. The particle size of ZnO is about 4.33 nm at the reaction condition of 50°C for 10 min and the photocatalytic performance is the best. The degradation rate of C.I. Reactive Blue 4 could exceed 75% in 30 min at room temperature when the initial concentration of C.I. Reactive Blue 4 was 40 mg/L and the concentration of the quantum-sized ZnO was 0.1 g/L.