ZnS thin films with a c-axis preferred orientation have been prepared by sulfurizing the RF reactive sputter deposited ZnO thin films at 500 degrees C in sulfur vapor. These thin films have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible transmission spectra and Auger electron spectroscopy (AES). It is found that the degree of c-axis Orientations of ZnO and ZnS thin films increases with enhancing RF power. Also, the sulfurized thin films have markedly greater grains than the as-deposited films, mainly due to recrystallization of ZnS in sulfur vapor. The grain size for the ZnS films increases from similar to 200 to similar to 500 nm with enhancing RF power from 80 to 120W. A reason for this is that in the case of the greater RF power, high compactness of the films will be very beneficial to the grain growth. Additionally, the excess sulfur can be absorbed on the grain surface of the ZnS film produced by 22 h sulfidation. This will cause the great broadening of absorption edge in the optical transmittance. (C) 2008 Elsevier B.V. All rights reserved.