Undoped and Cr doped ZnS nanoparticles with Cr concentrations of 3.0 at.% were prepared by a chemical co-precipitation method for the fist time, using 2-Mercaptoethanol as the capping agent and annealed the synthesized particles at 600°C for 3h in air. The effect of annealing on morphological, structural and optical properties of ZnS and ZnS:Cr have been studied and compared with as prepared samples. EDAX measurements confirmed the presence of Cr in the ZnS lattice and it also confirms the conversion of ZnS into ZnO after annealed at 600 0C/3h. Surface morphologies of all samples were characterized using scanning electron microscopy (SEM). XRD spectra of as synthesized nanoparticles of ZnS and ZnS:Cr exhibited cubic phase. After annealing, the cubic phase is transformed into hexagonal phase. The particle sizes of the ZnS:Cr powders were increased from 5 to 30 nm when the powders were annealed at 600°C. A stable blue emission peak at 445 nm is observed from the as prepared samples (pure ZnS and Cr doped ZnS) but annealed at 600 0C the PL peaked at 500 nm for pure ZnS and Cr doped ZnS nanoparticles exhibited PL peak at 500 nm as well as 654 nm. The emission intensity decreased in annealed particles compared to as synthesized samples.