n-ZnO thin films were deposited electrochemically onto the p-GaN/Al2O3 substrate in order to form hetero p-n junction. X-ray diffraction measurement has been showed clearly (0002) c-axis orientation of grown ZnO thin film. Absorption measurements were carried out before and after growth process indicating both sharp absorption edges of GaN and ZnO thin films. Photoluminescence measurement shows n-ZnO film grown on p-GaN has a dominant emission at 2.8 eV. I-V characteristic of n-ZnO/p-GaN/Al2O3 heterojunction showed that almost five order of rectification has been achieved. Turn on voltages of the p-n heterojunction is found to be 1.12 V.