High-quality ZnS epitaxial layers have been grown on (100)-oriented GaAs substrates by low-pressure metalorganic chemical vapor deposition. High-resolution X-ray diffraction and photoluminescence measurements were performed in order to characterize crystalline quality. The full-width at half-maximum (FWHM) of diffraction curves decreased with increasing layer thickness. The FWHM of the (400) diffraction curve (omega /2 theta -scan) and rocking curve (omega -scan) for an 8-mum-thick ZnS layer was as narrow as 60 and 125 arcsec, respectively. On the other hand, the photoluminescence spectrum was dominated by the radiative recombination of free excitons as a result of the reduction of residual impurity contamination. Moreover, the increase in the decay-time constant of the light hole free-exciton line with increasing layer thickness was observed, and the decay-time constant of light-hole free-exciton line for an 8-mum-thick layer was about 130 ps. These results directly indicate the decrease in nonradiative recombination centers with increasing layer thickness.