In this paper, the interface states of the AlGaN/GaN metal–insulator–semiconductor (MIS) high electron mobility transistors (HEMTs) with an Al2O3 gate dielectric are systematically evaluated. By frequency-dependent capacitance and conductance measurements, trap density and time constant at Al2O3/AlGaN and AlGaN/GaN interface are determined. The experimental results reveal that the density of trap states and the activation energy at the Al2O3/AlGaN interface are much higher than at the AlGaN/GaN interface. The photo-assisted capacitance-voltage measurements are performed to characterize the deep-level traps located near mid-gap at the Al2O3/AlGaN interface, which indicates that a density of deep-level traps is lower than the density of the shallow-level states.