Polycrystalline films ZnO has been grown onto glass substrates by chemical spray pyrolysis (CSP) method. They were given heat treatment at different temperatures and constant time and for different times with constant temperature in air. The change in structural and optical properties was studied by means of X-ray diffraction (XRD), SEM, and optical absorption measurements. Structural analysis by X-ray diffraction pattern showed annealed ZnO film has high-orientation along c - direction (0 0 2), which remained the same with different heat treatment. The lattice constants of ZnO thin films were also obtained from XRD data. It is found that, with the increase of different heat treatments, the lattice constant a increases from 3.208 Å to 3.254 Å, and c increases from 5.125 Å to 5.219 Å. Where at higher annealing temperature and time the lattice constant c and a approach from bulk value. Other orientations corresponding to (1 0 0) and (1 0 1) are presented with very low relative intensities as compared to that of (0 0 2) plane. The transparency is increasing with increasing annealing temperature and time due to decreasing in films thickness with increasing annealing temperature and time. Change in bandgap energy from 3.2 to 3.01 eV was observed for different heat treatments.