Gallium oxide (Ga2O3) films were deposited on MgO (100) substrates by metalorganic vapor phase epitaxy. Structure analyses showed that the films deposited at 550-700 degrees C were epitaxial beta-Ga2O3 films with an out of plane relationship of beta-Ga2O3(100)parallel to MgO(100). The film deposited at 650 degrees C showed the best crystallinity and the microstructure of the film was investigated by high resolution transmission electron microscopy. A theoretical model of the growth mechanism was proposed and the in-plane epitaxial relationship was given to be beta-Ga2O3[001]parallel to MgO < 011 >. A four-domain structure inside the epitaxial film was clarified. The beta-Ga2O3 film deposited at 650 degrees C showed an absolute average transmittance of 95.9% in the ultraviolet and visible range, which had an optical band gap of 4.87 eV. (C) 2012 Elsevier B.V. All rights reserved.