Reaction of a selenium alkaline solution with a nickel complex aqueous solution produces nanocrystalline pyrite NiSe2 with an average particle size of about 20 nm in the temperature range of 90-100 degrees C through a hydrothermal method. X-ray diffraction analysis indicates that the product is a single phase of pyrite nickel diselenide. TEM and ED images show that the product has a good crystallinity with a plate-like form. The synthesis can be performed conveniently and safely.