We have investigated the electrical properties of ZnOGaN heterostructures by capacitance-voltage (C-V) measurements. ZnOGaN heterostructures are fabricated on Ga-polar GaN templates by plasma-assisted molecular-beam epitaxy. The ZnOGaN heterostructures exhibit a plateau region of 6.5 V in the C-V curves measured at 10 kHz and room temperature. Moreover, it is found that a large electron density is accumulated at the interface of ZnOGaN, where the concentration approaches ∼ 1018 cm-3. The distinct C-V characteristics are ascribed to large conduction-band discontinuity at the ZnOGaN heterointerface. It is suggested that the ZnOGaN heterostructure is a very promising material for the application to heterojunction transistors.