The structural properties of a series of thin (0.2–0.6 &mgr;m) GaN epilayers directly grown on 6H–SiC(0001) by plasma-assisted molecular beam epitaxy are studied. X-ray reciprocal space maps of the GaN(0002) reflection reveal negligible inhomogeneous strain within the layer but a comparatively large orientational spread of the GaNcaxis. X-ray rocking curve measurements show, however, that this mosaicity steadily decreases with film thickness. In fact, the density of threading defects detected by transmission electron microscopy is found to decrease drastically with the distance away from the GaN/SiC interface, finally reaching a value of less than5×109&hthinsp;cm−2at a layer thickness of 0.5 &mgr;m. The formation mechanisms of the threading dislocations in the GaN films are discussed in consideration of the specific GaN/SiC interface structure. ©1998 American Institute of Physics.