Abstract: High-resolution X-ray diffraction was used to analyze the GaN layers grown on sapphire by metal-organic chemical vapor deposition（MOCVD）.The crystal structures and dislocation densities were determined by ω-scans of the different grazing incidence angle and skew diffraction.A new X-ray diffraction method for measuring longitudinal dislocation density of GaN thin film was proposed.The full-width at half-maximum（FWHM） of the（002） scanning curve of the sample are respectively 229.8arcsec、225.7 arcsec,indicating that the GaN films are strongly c-oriented.The samples show respectively threading dislocation densities of 4.0801×10⁸/cm²,5.8724×10⁸/cm²,as extracted from X-ray ω-scans in skew geometry.The dislocation densities of sample A are lower than that of sample B,which are inconsistent with luminescence results,while the dislocation densities deduced from grazing-incidence X-ray diffraction are consistent with luminescence results.