Nanoparticles of I-III-VI semiconductors are promising candidates for novel non-toxic fluorescent materials. However, removal of defect levels responsible for their broad-band emission has not been successful to date. The present study demonstrates, for the first time, the coating of core AgInS 2 nanoparticles-one of the I-III-VI group semiconductors with a bandgap in the visible region-with III-VI group semiconductors. The AgInS 2 /InS x and AgInS 2 /GaS x (x = 0.8-1.5) core/shell structures generate intense narrow-band photoluminescence originating from a band-edge transition at a wavelength shorter than that of the original defect emission. Microscopic analyses reveal that the GaS x shell has an amorphous nature, which is unexpected for typical shell materials such as crystalline lattice-matching ZnS. Singleparticle spectroscopy shows that the average linewidth of the band-edge photoluminescence is as small as 80.0 meV (or 24 nm), which is comparable with that of industry-standard II-VI semiconductor quantum dots. In terms of photoluminescence quantum yield, a value of 56% with nearly single-band emission has been achieved as a result of several modifications to the reaction conditions and post-treatment to the core/shell nanoparticles. This work indicates the increasing potential of AgInS 2 nanoparticles for use as practical cadmium-free quantum dots.