In this work, ZnO nanocrystalline thin films were prepared by sol–gel dip-coating method on glass substrates and the effect of substrate temperature on the microstructure and optical properties of these films was investigated. The structural properties were analyzed by X-ray diffraction (XRD) and scanning probe microscope. The optical properties were studied by UV-visible spectrophotometer and fluorophotometer. The experimental results showed that if substrate temperature was relatively high, once the substrate was dipped into ZnO sol, some aggregated colloidal particles would be absorbed on it. After the ZnO gel film was annealed, the poor quality ZnO thin film was formed, which had a weak ultraviolet emission and a relatively strong defect-related visible emission.