Zinc oxide (ZnO) thin films were deposited on the glass substrates using Zinc acetate dehydrate solution through spray pyrolysis method. The films were obtained by spraying 0.05M concentrated solution at a rate of 0.5ml for 10 min and were deposited at different substrate temperatures ranging from 473K to 673K. A comprehensive study was carried out to realize the effect of substrate temperaturesand subsequent annealing on optical properties of the ZnO thin films. It was observed that the band gap decreases with increases the substrate temperature. Optical constants such as the refractive index n and extinction coefficient k were determined from transmittance spectrum in the ultraviolet-visible (UV-VIS) regions by Swanepoel envelope method and crystallinity was confirmed by Urbach tail and atomic force microscopy images.