Hexagonal GaN epilayers with different AlxGa1-xN and AlN buffer layers were grown on Si(111) by metal-organic vapor phase epitaxy (MOVPE). Under the same growth conditions, the sample with four AlxGa1-xN buffer layers and one AlN buffer layer were grown on Si(111). According to the results of Rutherford backscattering (RBS)/channeling along axis, the conventional θ—2θ scans normal to GaN(0002) and (1122) plane at 0° and 180° azimuthal angles, and the reciprocal-space X-ray mapping (RSM) on GaN(1015) plane, we obtained that the crystal quality of the GaN epilayer is perfect with χmin＝1.54%. The crystal lattice constant of AlN, AlxGa1-xN and GaN epilayer has been calculated accurately, the lattice constant of GaN epilayer is almost equal to the theoretic data (aepi=0.31903nm, cepi=0.51837nm). The tetragonal distortion along the depth can got clearly from elastic strains of each layer in the normal and parallel directions, and the tetragonal distortion of GaN epilayer is nearly fully relaxed(eT=0).