ZnO thin films were deposited on microscope glass substrates by ultrasonic spray pyrolysis technique. The effects of annealing under various temperatures on the optical and structural properties of ZnO thin films were analyzed. The as-deposited and annealed ZnO thin films were investigated by UN/VIS spectrophotometer and X-ray diffractometer. Some of the optical properties of the films such as transmittance, absorbance and band gap energy were investigated by UV/VIS spectrophotometer. The crystallinity levels of the films were investigated, the structural parameters such as diffraction angle, full-width at half maximum, lattice parameters, grain size and dislocation density were calculated and structural properties were analyzed. X-ray diffraction patterns indicated that the ZnO films had a polycrystalline hexagonal wurtzite structure.