An efficient approach to engineering the Al2O3/GaN positive interface fixed charges by post-dielectric annealing in nitrogen is demonstrated. The remarkable reduction of interface fixed charges 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 significantly reduced interface fixed charges and the corresponding remote scattering effect enable respectable improvement in the electron mobility that results in a high drain current density of 355 mA/mm in the device. These competitive results reveal that the method reported in this letter is promising in pushing VTH more positive and simultaneously achieving good device performance of normallyoff GaN power devices.