An efficient approach to engineering the Al2O3/GaN positive interface fixed charges (Qit+) by postdielectric annealing in nitrogen is demonstrated. The remarkable reduction of Qit+ from 1.44×1013 to 3×1012 cm-2 was observed, which leads to a record high threshold voltage (VTH) of +7.6 V obtained in the Al2O3/GaN MOSFETs. The positive interface charges were proposed originating from the N-vacancy and Osubstitution at the Al2O3/GaN interface by Ab inito study. The significantly reduced Qit+ also effectively depresses the remote scattering effect that enables respectable improvement in the electron mobility, which results in a high drain current density of 355 mA/mm in the device with dimensions of LG/LGS/LGD/WG=2/1.5/5/50 ȝm. The device with LGD of 20 ȝm delivers a high breakdown voltage of 1054 V @ 1 ȝA/mm. Owing to the uniquely high VTH the fabricated normally-off device features substantially improved faulty turn-on immunity compared with the device with lower VTH. These competitive results reveal that the method reported in this work is promising in pushing the VTH more positive and simultaneously achieving good device performance of normally-off GaN power devices with improved fail-safe capability.