We report control of ZnO/GaN heterointerface by employing pre-treatments prior to ZnO films growth. Interface layer formation at ZnO/GaN heterointerface and control of interface laver formation was investigated. Interface layer was formed by first oxygen pre-exposure about 2-8 min to the Ga face (0001) GaN/sapphire substrate. The interface layer was observed directly by high-resolution transmission electron microscopy (HRTEM) and identified as a single crystalline monoclinic Ga2O3. ZnO film on the interface layer was single crystal. The formation of the oxide interface layer was prevented by first pre-exposure of zinc. Over-exposure (about 15 min) to oxygen plasma results in amorphous layer evaluated by reflection high-energy electron diffraction (RHEED) observation. Even on this surface, further ZnO films were grown as a single crystal as confirmed by RHEED and HRTEM observations. However, amorphous layer on the top of the interface layer was not observed by HRTEM. We concluded that the amorphous layer might be transformed to crystalline layer during ZnO growth by solid phase epitaxy.