Enhancement-mode (E-mode) AlGaN/GaN HEMTs with large gate swing and high threshold voltage is demonstrated in this work. The E-mode GaN HEMTs were fabricated by low energy fluorine ion implantation technique. Low energy fluorine ion implantation can provide higher fluorine ion concentration in the AlGaN layer and modulate the fluorine ion distribution in the GaN layer. The ion distribution can be also calculated by SRIM simulation. (The Stopping and Range of Ions in Matter) Without using gate insulator, the enhancement-mode GaN HEMT shows high threshold voltage of 2.5 V, with a current density of 200 mA/mm at a gate bias of 5.5 V. To confirm the implantation damages were removed, thermal stability test was performed in this work. After annealing at 400 °C for 10 minutes, the implanted devices showed no obvious threshold voltage shift.