Zinc oxide films were deposited on Si (111) substrates by radio-frequency (rf) sputtering at a room temperature and annealed in H2O, air, N2 ambient at temperatures between 700, 800 and 900 oC for 2 hrs. The effects of thermal annealing on the structural and optical properties of ZnO films were investigated by atomic force microscope (AFM), X-ray diffraction (XRD), photoluminescence (PL), Auger and transmission electron microscopy (TEM).