The annealing effect on the optical properties of ZnO epilayers was reported. The ZnO thin films were grown using metal organic chemical vapor deposition method, and annealed in atmosphere at 850 degrees C for different time periods. Double-crystal X-ray diffraction showed that, after annealing, the density of the edge-dislocations decreased dramatically upon annealing, however, the screw-dislocations were not affected significantly. Strong green photoluminescence was observed in the annealed ZnO films while the as-grown films showed only near band edge emission. The deterioration of the optical properties in the annealed film, however, only occurred near the film surface in a very thin layer. (c) 2005 Elsevier B.V. All rights reserved.