Gallium nitride (GaN) films were grown with and without lattice-matched (In,Ga)N buffer layers on the c(+) and c(-) faces of ZnO single-crystal substrates using molecular beam epitaxy, and their interface structures, including the relationship between the crystallinity of the GaN film and the polarity of the ZnO substrate, were investigated. Growth at a high temperature (e.g., 800°C) was made possible by using an (In,Ga)N buffer layer, which improved the crystallinity of the GaN films compared with that of GaN films grown directly on a ZnO substrate. Ion scattering, i.e., coaxial impact-collision ion scattering spectroscopy, and electron beam channeling, i.e., convergent beam electron diffraction, profiles revealed that the GaN films grown on c(+)- and c(-)-ZnO substrates with an (In,Ga)N buffer layer had a (0001) surface (i.e., c(+) polarity). That is, polarity inversion occurs when GaN film is grown with an (In,Ga)N buffer layer on a c(-)-ZnO substrate.