We report on the growth of polarity-controlled ZnO epilayers by plasma-assisted molecular-beam epitaxy and the measurement of valence-band offset at the ZnO/GaN heterointerface. The polarity of ZnO epilayers is determined by coaxial-impact-collision ion-scattering spectroscopy. The band offset is determined by ultraviolet and x-ray photoelectron spectroscopy. The high-resolution transmission electron microscopy study reveals the formation of an interface layer between the ZnO and GaN epilayers in O-plasma preexposed samples, while no interface layer is formed in Zn preexposed samples. Zn preexposure prior to ZnO growth results in Zn-polar ZnO epilayers (Zn face), while O-plasma preexposure leads to the growth of O-polar ZnO epilayers (O face). The interface layer is identified to be single-crystalline, monoclinic Ga2O3. The estimated valence band offset at the ZnO/GaN(0001) heterojunction with Zn preexposure is 0.8 eV with a type-II band alignment.