In reliability studies, the instability of the threshold voltage (Vth) is problematic when Vth is used as an indicator as it totally blurs an eventual drift due to real device aging. This instability is observed during electric characterization measurements and is related to the “biasing history” of the transistor which can introduce carrier trapping/detrapping in different layers of the structure. New methods are therefore required to overcome this trapping related instability issues in order to accurately monitor device aging. To solve the repeatability issue of threshold voltage measurement, we investigated its instability on GaN transistors. A preconditioning step applied right before the actual Vth measurement was studied. The proposed preconditioning method is based on the application of a dedicated Vgs (t) biasing on the gate terminal which leads to a stable and repeatable value of Vth. The mechanisms enabling the observed stability of Vth are identified through the analysis of the drain leakage measurement after the preconditioned Vth measurement. It demonstrates the role of hole injection into the structure. The preconditioned-Vth measurement method is proposed as a complementary measurement to correctly follow the aging of pGaN HEMT in future reliability studies.