Epitaxial growth of GaN has become an interest topic in term of light emitting device fabrication. Most of the commercial GaN based device is normally grown on sapphire substrate. For power device application, SiC has been found to be a desirable candidate for GaN epilayer due to their high thermal conductivity, small lattice mismatch, and hexagonal lattice mismatch with cleaved facet for the laser cavity. In this paper, X-ray diffraction (XRD) technique is employed to study the structural properties of GaN thin film grown on 6H-SiC substrate. For conventional XRD -2 scan, only diffraction peaks from GaN (002) and its multiple diffractions were observed, along with diffractions from SiC (006) peak. These results suggested that the GaN film is in wurtzite phase. For XRD rocking curve of omega scan of (002) diffraction plane of the GaN, a full width at half maximum of about 259 arcsec is obtained.