Objective(s): A large ratio of surface to volume of nanoparticles in comparison with bulk ones, will increase the cell penetration and therefore their toxicity. Materials and Methods: Chemical precipitation method was used in order to synthesis of ZnS:Ag quantum dots. Their Physical properties and characteristics were assessed by X-ray diffraction, Ultra Violet-Visible Spectrophotometer, Transmission Electron Microscope and it was shown that the obtained ZnS:Ag quantum dots are cubic with high-quality. Antibacterial effects of ZnS:Ag nanoparticles against Pseudomonas aeroginosa, Staphylococcus aureus and Salmonella typhi were investigated. Disc bacteriological tests were used in order to assessment of the antibacterial effects of ZnS:Ag nanoparticles. Results: The size of inhibition zone was different according to the type of bacteria and the concentrations of ZnS:Ag QDs. The maximum diameter was happened for S. aureus. The results of  MICs obtained fromBroth Dilution for Pseudomonas aeruginosa , Staphylococcus aureus and Salmonella typhi, are 3.05 , 3.05 and 6.1 mg/ml whereas the amounts of obtained MBCs are 12.2 , 6.1 and 12.2 mg/ml respectively. Conclusion: In conclusion, by increasing the nanoparticle concentration in wells and discs, the growth inhibition and diameter of inhibition zone has also been increased.