The localized states in ZnO nanowires (NWs) through the growth of ZnS shell have been introduced in this paper. Morphology and optical properties of the ZnO/ZnS core-shell heterostructured NWs after different rapid thermal annealing (RTA) treatments are investigated. Transmission electron microscopy measurements show the gradual disappearing of the jagged boundary between ZnO and ZnS with the increase of RTA temperature, while a decrease of interfacial composition fluctuation and a formation of ZnOS phase can be found after a RTA treatment of 300°C. Temperature-dependent photoluminescence exhibits the features of “S-shape” peak positions and a “valley shape” for the emission width, implying the existence of localized excitons in the core-shell NWs. Moreover, it is noted that the RTA treatments can lower the localized degree which is confirmed by optical measurement. The results indicate that the optical behavior of excitons in ZnO/ZnS core-shell heterostructured NWs can be manipulated by appropriate thermal treatments, which is very important for their practical device applications.