ZnS thin films have been deposited on glass substrates by RF magnetron sputtering. The effect of in-situ annealing in argon atmosphere at temperatures ranging from 300℃ to 500℃ on the structural and photoluminescence properties has been investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) spectroscopy. It has been found that the annealing has little effect on the crystallinity and grain size of ZnS films, but affects the photoluminescence significantly. The PL spectra of the films annealed at lower temperatures show a multiple-peak structure, while only single luminescence peak is observed for the sample annealed at 500℃. The difference in the PL spectra may result from the variation of the defect type and density in the ZnS films aroused by different annealing temperatures.