We report on growth and characteristics of a-plane GaN/ZnO/GaN epitaxial structures, which can be applied to various optoelectronic devices. The unique optical transitions intrinsic to heterovalent interfaces were found and analyzed. Clear carrier localization effect in the GaN/ZnO heterointerface is observed from the S-shaped energy shift with increasing temperature in the temperature-dependent photoluminescence measurement. The carrier localization also results in strong luminescent intensity and dominates the emission spectrum at room temperature. In addition, the acceptor level originated from the Zn out-diffusion from the ZnO layer is observed in the low temperature photoluminescence spectra, and its binding energy is estimated to be about 0.311 eV.