γ-irradiation technique was used with success to prepare nanocrystalline zinc sulfide at room temperature and ambient pressure, without adding any kind of catalysts. Sodium thiosulfate and carbon disulfide were used as sulfur sources in water and absolute alcohol systems, respectively. Structure and morphology of the nanocrystallines ZnS have been characterized by XRD and TEM analysis. The product has a cubic structure and, morphologically, is in spherical shapes of 20 to 50 nanometers in size. Probable mechanisms of the nanocrystalline formation in different systems are proposed in accord with the experiment.