Changes of growth behavior during heteroepitaxial growth of ZnO thin films on Al2O3 (0 0 0 1) substrates by pulsed laser deposition were investigated mainly using synchrotron X-ray scattering. In very early stages of growth (<= 130 angstrom in thickness), the films consist of only 2-dimensional (2D) ZnO (0 0 0 2) layers, which are well aligned to the Al2O3 (0 0 0 1) substrate. The 2D layers grow epitaxially with a 30 degrees rotation of ZnO basal planes with respect to the substrate. On top of the existing well-aligned 2D layers, poorly aligned 3-dimensional (3D) islands start to grow as the film growth proceeds further, while possessing significant amount of domains aligned with the hexagon-on-hexagon growth relationship. The onset of the 2D-3D transition, presumably occurring at 130-300 angstrom in thickness, is likely to be associated with the evolution of the double domains comprising 30 degrees rotated and hexagon-on-hexagon domains at that growth stage. (c) 2005 Elsevier B.V. All rights reserved.