The threshold voltage (VT) instability of metal‐insulator‐semiconductor high electron mobility transistors (MIS‐HEMTs) is investigated. In the enhancement‐mode AlGaN/GaN MIS‐HEMT fabricated by fluorine plasma implantation technique featuring Al2O3 gate dielectric, a hysteresis of 0.6 V in transfer characteristics is observed by quasi‐static I ‐V (current‐voltage) measurement, in which the gate bias is swept at a low rate (0.7 V/s). Pulsed transfer characteristics measurement, however, unveils much larger hysteresis under the same gate voltage swing. The VT‐instability is attributed to the traps located at the dielectric/III‐nitride interface as well as in the bulk of gate dielectric. It is proposed that pulsed measurement can count for the effect of fast traps (shallow traps) while the quasi‐static measurement can only reflect slow traps (deep traps), and is more accurate for VT‐instability evaluation. In addition, the existence of bulk traps is implied by a slow time‐dependent shift of VT under large gate bias.