Semiconductor nanoparticles of ZnS:Co are synthesized by wet chemical method at room temperature. The effects of increasing cobalt ions dopant in PL ZnS particles are studied, and the distribution, absorption, excitation, emission, and structural properties of particles have been determined. XRD investigation shows the cubic structure of ZnS nanoparticles and doping has no effect on the structure and the average crystalline particle size of the doped and undoped ZnS nanometerscale samples is about 2.5 nm. TEM shows the 1–3 nm size for ZnS:Co nanoparticles. UV-visible is used to study the optical absorption of ZnS pure as well as doped nanoparticles. Furthermore, as the chemical parameters change, the size of quantum dots changes systematically. Of course, doping percentage is a significant parameter in photoluminescence properties of quantum dots, but it has no effect on the size. The maximum intensity of photoluminescence spectra obtains the at 0.1% molar of the Co dopant in ZnS:Co (Ex = 283, Em = 485.423 nm) that shows the quenching point. The intensity of the emission bands decreases with the increasing of the doping percentage continuously, which indicates that cobalt ions are the eliminators of the photoluminescence.