Abstract: The crystal structure of the oxide film of Zr-4 alloy of different positions from the bottom of fuel rod, which comes from Qinshan nuclear power plant phase 1, is investigated by Raman spectroscopy method. The results indicate that the bottom of the rod has better corrosion resistance and more tetragonal ZrO₂, and has the black appearance which is compact. With the increasing of the distance from the bottom, the corrosion is more and more severe, the average content of tetragonal ZrO₂ in the oxide film decreases, that of monoclinic ZrO₂ increases, and tetragonal ZrO₂ transforms into monoclinic ZrO₂, and there is a gradual transition from the black and white appearance into white and porous appearance. In the cross section, from the oxide/metal interface to the surface of the oxide film, the content of tetragonal ZrO₂ reduces gradually and the monoclinic ZrO₂ increases. The result is similar to the investigations in the autoclave, i.e., the transformation from tetragonal ZrO₂ to monoclinic ZrO₂ decides the corrosion resistance of Zr-4 alloy in the reactor, and the higher the m-ZrO₂ content in the oxide film is, the higher corrosion rate of cladding, and the worse corrosion resistance property.