Selective area growth of GaN nanopillars was performed via gas source molecular beam epitaxy using ammonia gas as a nitrogen source and the associated growth mechanism was discussed. The aspect ratio of the pillar structures was found to be sensitive to both growth temperature and V/III ratio. The primary reason for the change in aspect ratio is re-evaporation of nitrogen from the growth surface. In addition, the growth condition under which the pillar diameter was dependent on only the window diameter of the selective growth mask was clarified, and a maximum aspect ratio of 61 was obtained for a 9-nm pillar diameter. (C) 2002 Elsevier Science B.V. All rights reserved.