The CdSe/ZnS core-shell nanocrystals (NCs) are synthesized by a simple and low cost chemical route. The optical properties of the prepared samples are characterized by UV–Vis absorption spectroscopy, fluorescence spectroscopy, quantum yields (QYs) and Transmission electron microscopy (TEM). The studies show that CdSe/ZnS nanocrystals sizes at the first excitonic absorption peak increase with increasing time and temperatures. The spectroscopic study of the prepared CdSe/ZnS core-shell nanoparticles shows that the crystal size of the nanoparticles are in the range of 4.47-5.41nm. TEM results reveal that CdSe nanoparticles are well-ordered crystallized with average particle sizes 4.5-5.5 nm. Fluorescence analysis of the synthesized CdSe/ZnS NCs samples reveals that CdSe/ZnS NCs of the crystal size range 5.25-5.74 nm exhibit luminescence output peak around 550-570 nm. The homogenous formation of CdSe/ZnS NCs core-shell are confirmed by the maintained full-width at half maximum (FWHM) of fluorescence emission spectra for the samples prepared under different time. At the room temperature fluorescence QYs of CdSe/ZnS NCs are 27%-45%.