We have investigated the mechanism of phase transformation from ZnS to hexagonal ZnO by hightemperature thermal annealing. The ZnS thin films were grown on Si(001) substrate by thermal evaporation system using ZnS powder as source material. The grown films were annealed at different temperatures and characterized by X-ray diffraction(XRD), photoluminescence(PL), four-point probe, scanning electron microscope(SEM) and energy dispersive X-ray diffraction(EDX). The results demonstrated that as-deposited ZnS film has mixed phases but high-temperature annealing leads to transition from ZnS to ZnO. The observed result can be explained as a twostep process:(1) high-energy O atoms replaced S atoms in lattice during annealing process, and(2) S atoms diffused into substrate and/or diffused out of the sample. The dissociation energy of ZnS calculated from the Arrhenius plot of 1000/T versus log(resistivity) was found to be 3.1 eV. PL spectra of as-grown sample exhibits a characteristic green emission at 2.4 eV of ZnS but annealed samples consist of band-to-band and defect emission of ZnO at 3.29 eV and 2.5 eV respectively. SEM and EDX measurements were additionally performed to strengthen the argument. We have investigated the mechanism of phase transformation from ZnS to hexagonal ZnO by hightemperature thermal annealing. The ZnS thin films were grown on Si (001) substrate by thermal evaporation system using ZnS powder as source material. The grown films were annealed at different temperatures and characterized by X-ray diffraction (XRD), photoluminescence (PL), four-point probe, scanning electron microscope (SEM) and energy dispersive X-ray diffraction high-temperature annealing leads to transition from ZnS to ZnO. (1) high-energy O atoms replaced S atoms in lattice during annealing process, and (2) S atoms diffused into substrate and / or diffused out of the sample. The dissociation energy of ZnS calculated from the Arrhenius plot of 1000 / T versus resistivity was found to be 3.1 eV. PL spectra of as-grown sample exhibits a characteristic green emis sion at 2.4 eV of ZnS but annealed samples consist of band-to-band and defect emission of ZnO at 3.29 eV and 2.5 eV respectively. SEM and EDX measurements were additionally performed to strengthen the argument.