Metal-semiconductor-metal interdigital Schottky UV detectors are fabricated on unintentionally doped GaN single crystal grown by metalorganic chemical vapor deposition technique. The impact of low vacuum thermal annealing on the current-voltage characterization of devices has been studied based on thermionic emission theory. The barrier height of Au-GaN Schottky barrier changed from 0.36 eV(before annealing) to 0.57 eV (400 ℃ 0.5 h), but it reduced after the annealing time extended to an hour. Analysis results show that the defects caused by sputtering interstitial Au atoms make the barrier low. And the increasing of barrier is caused by N vacancies filled with Au atoms which act as donors after annealing.