This invention relates to a method for preparing a Zinc Sulfide powder, and provides a method for preparing a single crystalline powder of Zinc Sulfide (ZnS) a high crystallinity comprising a step of conducting a hydrothermal reaction of a) Zinc Oxide or Zinc acetate as Zinc source and b) thioacetamide or thiourea as Sulfur source at a temperature of 180 to 230.degree. C. and a fluorescent substance using the same as a source.