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Zinc oxide (ZnO) thin films were deposited on (100) Si substrates by sol-gel technique. Zinc acetate was used as the precursor material. The effect of different annealing atmospheres and annealing temperatures on composition, structural and optical properties of ZnO thin films was investigated by using Fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy and photoluminescence (PL), respectively. At an annealing temperature of 400°C in N2 for 2 h, dried gel films were propitious to undergo structural relaxation and grow ZnO grains. ZnO thin film annealed at 400°C in N2 for 2 h exhibited the optimal structure and PL property, and the grain size and the lattice constants of the film were calculated (41.6 nm, a = 3.253 A and c = 5.210 A). Moreover, a green emission around 495 nm was observed in the PL spectra owing to the oxygen vacancies located at the surface of ZnO grains. With increasing annealing temperature, both the amount of the grown ZnO and the specific surface area of the grains decrease, which jointly weaken the green emission.