ZnSxSe1-x (x < 0.18) random alloy and ordered alloy were grown on GaAs (0 0 1) substrate by metalorganic chemical vapor deposition in ALE mode (MOCVD-ALE) using dimethylzinc, H2Se and H2S as source materials. In order to investigate the strain effect of ZnSxSe1-x. epilayers, heavy hole (hhx) and light hole (lhx) exciton peaks in PL spectra were monitored as a function of S composition. The identification of hhx and lhx peaks was confirmed by photoreflectance spectroscopy. The S composition at which lattice matching took place was determined to be 5.6%. The full-width at half-maximum (FWHM) of hhx peak increased and deep-level emission appeared in the PL spcctra as the S composition increased. For the ordered alloy, the FWHM of exciton peak was narrower than that for the random alloy and the deep level emission was suppressed. An analysis of broadening of the exciton line show that the optical properties of random alloy are influenced by alloy potential and that the alloy potential fluctuation is significantly constrained in the ordered alloy.