Atomic layer deposition technique and subsequent rapid thermal annealing (RTA) were implemented to grow high-quality ZnO epilayers for the fabrication of n-ZnO/p-GaN heterojunction LEDs. The X-ray diffraction measurement reveals that the ZnO epilayer has high crystallinity with c axis orientation. Transmission electron microscopy images present that the ZnO layer is a single crystal, including only a few survivals of threading dislocations, which were generated in the GaN layer deposited by metal-organic chemical vapor deposition on the c-Al2O3 substrate and most of which were eliminated at the n-ZnO/p-GaN interface. An interfacial layer 4-5 nm thick caused by the RTA treatment was observed between the n-ZnO and p-GaN layers. Room temperature UV electroluminescence (EL) at 391 nm from ZnO was achieved at a low injection current about 10 mA. It is concluded that the competition between the ELs from the n-ZnO and p-GaN (around 425 nm) may be ascribed to the ZnO/GaN interface states coupled with the differences between the n-ZnO and p-GaN in carrier concentration and light emission efficiency.