This paper reports on the investigation of GaN growth by low-pressure metal organic chemical vapour deposition technique on silicon carbide (SiC) substrates. The critical impact of some growth parameters on the physical properties of the GaN epilayers has been identified and studied. using high resolution X-ray diffraction (HR-XRD) and transmission electron microscopy (TEM) measurements. The SiC substrate surface preparation (ex situ and in situ surface preparation: deoxydation. annealing temperature, etc.) and the GaN nucleation layer growth conditions (growth temperature, layer thickness, strain) have been found to be the key steps for the GaN/SiC growth. GaN epilayers grown on SiC substrates with a specific surface preparation (Novasic) have shown by HR-XRD full-linewidths at half-maximum of 50 arcsec. TEM measurements have evidenced an improved crystalline quality of the GaN/SiC interfaces at low growth temperature of the first GaN monolayers. showing steps originating from the surface misorientation. (C) 2002 Elsevier Science B.V. All rights reserved.