Growth of a ZnO/GaN heterostructure is carried out using pulsed laser deposition. By etching the ZnO layer from the ZnO/GaN structure, the photoluminescence (PL) of the associated GaN layer shows that the donor-acceptor luminescence of GaN shifts to about 3.27eV, which is consistent with the electroluminescence (EL) of n-ZnO/p-GaN already reported. XPS shows that oxygen diffuses into the GaN crystal lattice from the surface to 2Ohm depth. The PL spectra at different temperatures and excitation densities show that oxygen plays the role of potential fluctuation. The associated PL results of the interface in these LEDs could be helpful to understand the mechanism of EL spectra for ZnO/GaN p-n junctions.