ZnO thin films with good visible emissions were deposited on Al2O3 substrates by pulsed laser deposition and subsequently annealed at different temperatures in oxygen ambient. The visible emission property of the films varied significantly with different annealing temperatures. The resistivity, carrier concentration and mobility of the films showed certain rules. From the results of X-ray diffraction, scanning electron microscope, photoluminescence and Hall measurements, the mechanism of visible emission and the reason of showing n-type conductivity in native ZnO thin films were analyzed in this paper.