SiC was grown on the top Si layer of SIMOX by carbonization followed by chemical vapor deposition (CVD). Subsequently, GaN was deposited on the SiC by metalorganic (MO)CVD to produce a GaN/SiC/Si/SiO2/Si multilayer structure. This multilayer film was investigated by conventional transmission electron microscopy (TEM) and high resolution (HR)TEM using cross-sectional thin foils. The GaN layer was found to consist of predominately hexagonal gallium nitride (h-GaN), and a smaller fraction of cubic GaN (c-GaN) crystallites. The orientation relationship between most of the h-GaN grains and cubic SiC (3C-SiC) was found to be (0001)GaN//(1I1)sic; [1120]GaN//[110]SiC, while most of the c-GaN grains had an orientation relationship (001)GaN//(001)SiC; [110]GaN//[110]SiC with respect to the 3C-SiC substrate. The hexagonal grains of GaN were found to grow as two variants. In this paper, the defects in both h-GaN and c-GaN are characterized and discussed.