The optoelectronic properties of ZnS nanowires (NWs) and ZnS-Si core shell nanowires (CSNWs) oriented along different directions has been investigated using first principles calculations. Both NWs and CSNWs exhibit semiconductor characteristics in [100] and [111] directions while [110] oriented NWs and CSNWs exhibit metallic characteristics. The semiconductor characteristics of the ZnS NWs and ZnS-Si CSNWs in [100] and [111] directions have been confirmed from the electronic and optical energy gaps. We observed strong optical absorption in [110] directional ZnS-Si CSNWs throughout the spectrum from visible to UV region which represents significance to the optoelectronic and energy harvesting devices. Furthermore, the parallel and perpendicular parts of the optical absorption of ZnS-Si CSNWs show good improvement as compared to ZnS NWs. The refractive index and reflectivity spectra of ZnS-Si CSNWs also offered considerably upgraded values than ZnS NWs. Our predicated optical static dielectric constant epsilon(1)(0) and static refractive index n(0) values are also much better than the previously reported one.