Zinc oxide thin films are deposited on Si and quartz substrates using the sol–gel method. The thin films, annealed at 400, 600 and 800 °C respectively, are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), ultraviolet–visible spectrophotometer (UV–Vis), fluorescence spectrometer (FL) and the photocatalytic activity is tested by the decomposition of methyl orange dye under UV illumination. The results show that the mean grain size, surface-to-volume ratio, rms roughness and degradation efficiency of the thin films increases with increasing annealing temperature. In particular, ZnO thin film annealed at 800 °C exhibits the highest photocatalytic activity, degrading methyl orange by almost 88% in 180 min. Photocatalytic reaction mechanism of the ZnO thin films is discussed in detail, and the oxygen defects are proposed to be the active sites of the ZnO photocatalyst.