Abstract: A hexagonal GaN layer with a LT-AlN (low temperature) interlayer grown on Si(111) by metal-organic chemical vapour deposition (MOCVD) method was characterized by high resolution X-ray diffraction(HRXRD) and Rutherford backscattering (RBS)/channeling. The crystal quality of the film is perfect with a χ_min=2.5%. According to the results of XRD ω-scan, the angle between the GaN (0001) plane and the Si(111) plane was determined. The elastic strains in the perpendicular and parallel directions were calculated to be -0.10%±0.02% and 0.69%±0.09% respectively by θ-2θ scan of GaN(0002) and GaN(1015) diffractions. By the angular scan around an offnormal <1213> axis in the 1010 plane of the GaN layer, the tetragonal distortion e^T, which is caused by the elastic strain, was determined to be 0.35%±0.02% near the surface, indicating a tensile strain in the parallel direction (e^//>0) which results in a compressive strain in the perpendicular direction (e^⊥<0) due to the elastic properties of the epilayer. This result coincides with that from XRD perfectly. The elastic strain of the GaN epilayer under the AlN interlayer releases more quickly than which upon the AlN interlayer with the thickness increasing.