Hydrothermal treatment has been successfully used to prepare pure nanocrystalline ZnS from amorphous ZnS. XRD indicated that the product was cubic ZnS phase (Sphalerite). Electron microscopy image showed that the average particle size was about 6nm. As-prepared Sphalerite powder with narrow particle size distribution possesses good IR transmittance in the range of 400cm(-1) to 5000cm(-1).