Single crystalline ZnO thin films have been deposited on epi-GaN/sapphire (0001) substrates by the metalorganic chemical vapour deposition (MOCVD) method. The structural and optical properties of the ZnO films were investigated in detail. The film prepared at 600°C was epitaxial single crystalline with the wurtzite structure of pure ZnO and a single orientation of (0002) direction. High-resolution transmission electron microscopy was used to investigate the interface area, and a clear orientation relationship of ZnO (0001) || GaN (0001) and ZnO  || GaN  was obtained. The average transmittance for the samples in the visible range was over 75%.