X-ray diffraction (XRD) measurements of Phi scan in different chi angle in skew geometry for GaN films have shown that the peak widths of Phi scan decrease with the increment of angle chi. The FWHM of omega scan also increases with the inclination angle and reaches to be equal until the reflection plane is perpendicular to the surface of the sample. Based on these measured results, we developed a method to determine directly the twist angle of un-doped and Si-doped GaN films grown on c-sapphire substrates by MOCVD. (C) 2003 Elsevier Science B.V. All rights reserved.