Gallium oxide thin films of beta and epsilon phase were grown on c-plane sapphire using metal-organic chemical vapor deposition and the phase compositions were analyzed using X-ray diffraction. The epitaxial phase diagram was constructed as a function of the growth temperature and VI/III ratio. A low growth temperature and low VI/III ratio were beneficial for the formation of hexagonal-type epsilon-Ga2O3. Further structure analysis revealed that the epitaxial relationship between epsilon-Ga2O3 and c-plane sapphire is epsilon-Ga2O3 (0001) parallel to Al2O3 (0001) and epsilon-Ga2O3 parallel to Al2O3. The structural evolution of the mixed-phase sample during film thickening was investigated. By reducing the growth rate, the film evolved from a mixed phase to the energetically favored epsilon phase. Based on these results, a Ga2O3 thin film with a phase-pure epsilon-Ga2O3 upper layer was successfully obtained. (C) 2017 Elsevier B.V. All rights reserved.