The crystal orientation relationship between beta-Ga2O3 and MgO in beta-Ga2O3 thin films prepared on (1 0 0), (1 1 1), and (1 1 0) MgO substrates was investigated by X-ray diffraction measurements and crosssectional transmission electron microscopy images. The gamma-Ga2O3 interfacial layer was present between beta-Ga2O3 and MgO acted as a buffer to connect beta-Ga2O3 on MgO. The following conditions were satisfied under each case: beta-Ga2O3 (1 0 0)|| MgO (1 0 0) and beta-Ga2O3 [0 0 1]|| MgO < 0 1 1 > for the formation of beta-Ga2O3 on (1 0 0) MgO, and beta-Ga2O3 ((2) over bar 01)|| MgO (1 1 1) for the formation of beta-Ga2O3 on (1 1 1) MgO, as well as each condition of beta-Ga2O3 [0 1 0] ((1) over bar1 0 0)|| MgO [10] (0 0 1), beta-Ga2O3 [0 1 0] (1 0 0)|| MgO [0 (1) over bar1] (1 0 0), and beta-Ga2O3 [0 1 0] (1 0 0)|| MgO [10 (1) over bar] (0 1 0). beta-Ga2O3 (102)|| MgO(1 1 0) and beta-Ga2O3 [0 1 0] perpendicular to MgO [0 0 1] for beta-Ga2O3 formed on (1 1 0) MgO. The beta-Ga2O3 formed on (1 1 1) MgO at 800 degrees C exhibited a threefold structure. The beta-Ga2O3 formed on (1 1 0) MgO had a twofold structure but different by 90 degrees from the result reported previously. (C) 2017 Elsevier B.V. All rights reserved.