The preparation of ZnO thin films using the polymeric precursor method is reported, and the influence of the thickness and calcination temperature on their effective photocatalytic activity was analyzed. The ZnO thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance UV-vis spectroscopy (UV-DRS), and atomic force microscopy (AFM). Thicker films with a lower calcination temperature exhibited better photoactivity for RhB photodegradation under UV irradiation. The average particle size increased from 14 to 45 nm as the calcination temperature increased from 400 to 600 °C. In addition, the stability results indicated no significant deactivation even after four successive re-uses in RhB photodegradation.