To investigate the thermal preheating effect of the GaAs substrate exerted on the ZnS epilayers for the first time, ZnS epilayers were grown on the GaAs (100) substrate by hot wall epitaxy. The thermal preheating temperature was $450^{circ}C$~$660^{circ}C$. The full width at half maximum values of double crystal rocking curve were the smallest for the ZnS epilayers grown on the GaAs thermally preheated at around both $500^{circ}C$ and $600^{circ}C$. However, photoluminescence characteristics of ZnS epilayers were better at $600^{circ}C$ than at $500^{circ}C$. Therefore, it was shown that the optimum preheating temperature of the GaAs substrate for the growth of high quality ZnS epilayer was around $600^{circ}C$. From these experimental results, it was shown that the crystal quality and the PL properties of ZnS epilayers were enhanced for the GaAs substrates thermally preheated at $600^{circ}C$.