Manganese-doped zinc sulfide (ZnS:Mn) nanoparticles were synthesized by mixing aqueous solutions of zinc nitrate and manganese nitrate with sodium sulfide. The photoluminescence (PL) of ZnS:Mn nanoparticles (NPs) dispersed in the aqueous solution was observed at 590 nm using extinction light 320 urn in wavelength. When the S and Zn ion concentrations were equal (equimolar condition), the Mn content in NPs increased with the molar ratio Mn/Zn in the feed solution. PL intensity and PL quantum efficiency were proportional to the Mn content in NPs. The particle size of ZnS:Mn increased with the time elapsed after preparation, although the Mn content was constant. NP absorbance at 320 urn increased with elapsed time due to increasing NP size so that PL intensity increased because of increasing absorbed light energy. However, the quantum efficiency remained almost constant. When the S ion concentration exceeded the Zn ion concentration (excess S ion condition), the particle size and absorbance at 320 urn increased with elapsed time, exhibiting trends similar to those observed in NPs prepared under the equimolar condition. On the other hand, the quantum efficiency under an excess S ion condition increased with the elapsed time.