Nanocrystalline zinc sulfide particles have been prepared by the gas-phase condensation method. By this method the material is evaporated in the presence of a few Torr of Helium; so that, by collisions with the inert gas atoms, the evaporated atoms lose kinetic energy and condense in the form of ultrafine particles of a few nanometers in diameter. The obtained material has been characterized by XRD (X-Ray Diffraction), TEM (Transmission Electron Microscopy) and UV-Vis absorption spectroscopy. The ZnS ultrafine powder shows an homogeneous particle size distribution with a mean crystallite diameter of 8 nm. The quantum size effect on the band-gap absorption energy, as measured by UV-Vis absorption spectroscopy, has been correlated with the particle size of the nanostructured material.