GaN films were grown at various V/III ratios on sapphire (0001) substrates with and without nitridation process by radio-frequency plasma-excited molecular beam epitaxy. Based on the results of X-ray diffraction pole-figure and phi scan measurements, the GaN film grown without nitridation process has a tendency to form the metastable in-plane rotation domain by similar to11degrees from the Substrate, in addition to the main domain rotated by 30degrees from the substrate. In suppressing the formation of the metastable in-plane rotation domain, it is found to be very effective to perform the nitridation process. In addition, even in GaN Elms grown without nitridation process, decrease in V/III ratio enables us to suppress the formation of the metastable domain. The rotation domains are considered to have the epitaxial relationship of [12 (3) over bar0](GaN)parallel to[11 (2) over bar0](sapphire) and [(1) over bar(2) over bar 30](GaN)parallel to[11 (2) over bar0](sapphire). (C) 2002 Elsevier Science B.V. All rights reserved.