By a metastable rocksalt ZnO interfacial structure and a resulting rocksalt-to-wurtzite structure transition in ZnO buffer layer, polarity control of Zn-polar ZnO films was realized on rocksalt MgO (1 1 1) template by plasma-assisted molecular beam epitaxy. The metastable rocksalt ZnO and the phase transition from octahedrally bonded rocksalt to tetrahedrally bonded wurtzite structure during the growth of ZnO buffer layer were established by the combination of in situ reflection high-energy electron diffraction and ex situ high-resolution transmission electron microscopy. Such a metastable rocksalt ZnO has a lattice constant a = 4.16 angstrom and a typical ...abcabc... atom stacking along the growth direction. Polarity selection of Zn-polar ZnO films was found greatly depending on the bonding configuration of the epitaxial ZnO interfacial layer, and an atomic model of the structure transition was proposed to explain the mechanism of ZnO polarity selection. (C) 2009 Elsevier B.V. All rights reserved.