CuInS2 (CIS) nanoparticles (NPs) were hydrothermally prepared by the reaction of Cu2+, In2+, and Na2S in the presence of N-acetyl-L-cysteine as a stabilizer. The X-ray diffraction pattern of the resultant CIS NPs indicated that the chalcopyrite NPs could be obtained. The X-ray photoelectron spectroscopy data suggested that Cu2+ is reduced to Cu+ during the reaction process. The absorption and photoluminescence peaks of the NPs were clearly red-shifted with increasing size of the particles. Our method will aid in the synthesis of other I-III-VI ternary compounds.