ZnS thin films were prepared by sulfuring zinc thin films at different sulfuration temperatures. The crystal structure, surface morphology, defects, and optical properties of the thin films were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), positron annihilation Doppler broadening, and UV-Vis spectrophotometer, respectively. It was found that the (200)-plane preferred orientation of the ZnS thin films changed to (111)-plane with increasing sulfidation temperature. Moreover, a number of large holes were generated at 420 °C and eliminated at 440 °C. The concentration of defects was lowest when the sulfuration temperature was 440 °C. The optical transmission of all samples was maintained at 60%–80% in the wavelength range of 400 nm–800 nm, and the band energy of the ZnS thin films was approximately 3.5 eV for all treatment temperatures except 430 °C.