ZnS:Ag (Ag+-doped ZnS) nanoparticles have been synthesized through the hydrothermal route, and the effects of the molar ratio of S/Zn in reagents have been studied. The products are all cubic sphalerite structure, and the crystallinity is generally increased as this ratio rises. The ratio less than 1 leads to small particles less than 10 nm, and the ratio more than 1 leads to large particles between 20~30 nm. Luminescence properties of samples show significant dependence on the ratio. The sample synthesized using stoichiometric reagents during hydrothermal preparation exhibits the strongest blue emission in this series. There is significant defect luminescence in samples synthesized using nonstoichiometric reagents and the Ag-related luminescence is quenched in them.