Polycrystalline ZnO thin films have been prepared on glass substrates by spray pyrolysis technique. The deposition temperature was varied between 350°C and 570°C. The effects due to post-thermal annealing on the structure and optical properties of thin films have been studied depending on the annealing temperature in the range of 200 ÷ 550°C. The crystal structure and surface characteristics were investigated by mean of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The optical properties were studied by optical transmission spectrometry using a UV-VIS spectrophotometer. The optical measurements reveal that thin films have a maximum transmittance of about 86% and a direct band gap of 2.29 eV. XRD and SEM both indicate that increasing the annealing temperature results in an increase of the c-axis-oriented columns on the glass substrates.