Abstract: High quality chalcopyrite CuInSe2 films were synthesized by a non-vacuum solution method using nitrates as sources for low-cost photovoltaic applications. The solution based on nitrates was used for precursor films by the spin-coating process, and sequential reduction and selenization treatments were performed with hydrogen and Se vapor, respectively. Through X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscope (SEM) and Raman scatting measurements, it was found that uniform, high crystalline and crack-free films were obtained using this simple method. Also, the films exhibited Cu2-xSe secondary phase free property, indicating the excellent quality of the synthesized CuInSe2 thin films. Finally, the phenomenon and mechanism of the two-layered structure formation of the synthesized CuInSe2 thin film were discussed.