Abstract: An Al_xGa_1-xN film with a Al content of 0.63 was grown on the sapphire substrate by the metal-organic chemical vapor deposition. High resolution x-ray diffraction method can used to measure the crystal lattice constant of the Al_xGa_1-xN film. The lattice constant of the Al_xGa_1-xN film can be accurately determined by the ω/2θ scan of the symmetric crystal plane and asymmetric crystal plane and also by the correction of the zero error and distance of crystal planes. The result showed that the level lattice constant of Al_0.63Ga_0.37N is 0.505 96 nm,and the vertical lattice constant is 0.313 01 nm. Through the analysis and correction of various effects, the obtained measurement deviations are 0.000 1 nm and 0.000 2 nm,respectively.