Undoped ZnS epilayers were grown on (100)GaAs by molecular beam epitaxy (MBE). The epilayers had a granulated surface and their photoluminescence (PL) showed three peaks. The peaks could be recognized to be a near-band-edge emission, a defect-related emission and an As-related red luminiscence. Furthermore, the post-annealing at 500°C for 30 min made the shape of the granules unclear and reduced the intensity of the defect-related emission in PL.