The nanometer-sized zinc sulfide (ZnS) as a blue-emitting phosphor was prepared in a water-in-supercritical CO2 microemulsion. The nanoparticles were characterized by XRD, TEM, and PL. XRD analysis shows that the average crystalline size of ZnS nanometer-scale samples is about 2-3 nm. Emission intensity of the ZnS nanophosphors obtained by the process is higher than that micrometer-sized ZnS.