Crystal characterization by photoluminescence and x-ray diffraction has been performed to find out the optimum growth condition of ZnS grown by the gas-source molecular-beam-epitaxial (MBE) method. In this crystal growth, important parameters are the cracking temperature of H2S gas (Tcr), the film thickness (d), both molecular-beam flux intensities (JZn and JΣS or inlet H2S gas pressure Pin), and substrate temperature (Tsub). A single crystal having a fairly high quality, whose half width of x-ray diffraction FWHM is about 0.119° (the reference value of the GaAs substrate used is 0.093°), is obtained under the growth conditions Pin=1.4×10−5 Torr (corresponding flux intensity JΣS=7×1015 cm−2 s−1), JZn/JΣS=1, and Tsub=325 °C in the case of Tcr=780 °C and d≂1 μm.