The nano-particle pyrite in nano-particle form was synthesized by the deposition hydrothermal reaction using FeCl3•6H2O, Na2S•9H2O and S as the starting materials. The property of particles was investigated by X-ray diffraction, transmission electron microscope, UV-Vis spectrophotometer. The results indicated that the pure iron pyrite could be achieved in the alkaline condition. Increasing the proportion of S avails to removing marcasite and ferric oxide . Wwhen the amount of FeCl3, Na2S and S is in the ratio of 1:3:1.5, the product is pure pyrite FeS2 powder with a polyhedron shape is obtained and the average particle size isof about 20~40nm, and this result can be easily repeated. Optical analysis indicates that the optic energy band gap is about 1.62 eV. Its open circuit voltage is 456mV, and the corresponding short circuit current is 19 μA.