In this letter, a normally-off AlGaN/GaN MIS-HEMT using fluorinated gate dielectric was presented. The fluorine ions were injected into the Al 2 O 3 gate dielectric to obtain positive threshold voltage (V th ) as well as avoiding the plasma induced to the GaN channel layer. Moreover the maximum transconductance of fluorinated gate MIS-HEMT has been improved compared with the non-treated MIS-HEMT. Furthermore, the fluorine ions injected into the Al 2 O 3 gate dielectric could decrease the trap states density (D T ) and time constant (τ T ) at the Al 2 O 3 /GaN interface. The normally-off MIS-HEMT showed a very high drain current of 507 mA/mm and V th of 0.6 V.