Zinc oxide thin films fabricated on si (111) and sapphire substrates by pulsed laser deposition (PLD) were annealed at different temperatures and in different ambient, respectively. The effects of annealing temperature and annealing ambient on structural and optical properties of ZnO thin films have been characterized by Xray diffraction (XRD) and photoluminescence (PL) spectra. The experimental results show that with the increase of annealing temperature the compressive stress of ZnO thin films is decreased and shifted towards strain stress. PL spectra of ZnO thin films annealed at different temperature indicate that the UV emission is increasing with increasing annealing temperature, and the visible emission is decreasing. The results annealed at various conditions show that the thin film annealed on sapphire substrate at 700℃ in oxygen ambient possesses a small strain stress and the highest ratio of UV emission to visible emission.