We experimentally evaluated the interface state density of GaN MIS-HEMTs during time-dependent dielectric break-down (TDDB). Under a high forward gate bias stress, newly increased traps generate both at the SiNx/AlGaN interface and the SiNx bulk, resulting in the voltage shift and the increase of the voltage hysteresis. When prolonging the stress dura-tion, the defects density generated in the SiNx dielectric becomes dominating, which drastically increases the gate leakage current and causes the catastrophic failure. After recovery by UV light illumination, the negative shift in threshold voltage (compared with the fresh one) confirms the accumulation of positive charge at the SiNx/AlGaN interface and/or in SiNx bulk, which is possibly ascribed to the broken bonds after long-term stress. These results experimentally confirm the role of defects in the TDDB of GaN-based MIS-HEMTs.