GaN epilayers have been grown on (11·0) sapphire substrates in a low-pressure MOCVD system which permits the rotation of the samples during the growth. When the substrates were rotating, high quality GaN has been obtained and the crystal was only in the (00·1) orientation. When there was no rotation, two different orientations of GaN have been observed: (00·1) and (10·0), and the crystalline quality deteriorated. X-ray diffraction and precession photographs yielded the in-plane direction relationship between the epilayer and the substrate. Electron microscopy showed that the layer was a mixture of both crystal phases, suggesting it was a highly-oriented polycrystal. A simple crystallographic modeling of the growth of (00·1) GaN and (10·0) GaN on (11·0) Al2O3 has been developed to interpret the formation of both orientations of GaN.