Early stage growth behavior of vertically aligned ZnO nanoneedles by metalorganic chemical vapor deposition was investigated mainly using X-ray diffraction. In very early stages of growth, 3-dimensional island-like ZnO grains initially nucleate and grow on the substrate surface, then cover its whole surface, consequently resulting in formation of a continuous layer. The layer grows epitaxially with a 30 degrees rotation of ZnO basal planes with respect to the Al2O3 (0 0 0 1) substrate, while being coherently strained to the substrate. On top of the existing well-aligned continuous ZnO layer, vertically as well as laterally aligned nanoneedles start to grow as the growth proceeds further. The onset of the nucleation and growth of nanoneedles is likely to be associated with the relaxation of a compressive strain developed in the continuous ZnO layer formed before the growth of nanoneedles. (c) 2005 Elsevier B.V. All rights reserved.