We have studied numerically the capacitance properties of insulator/AlGaN/GaN MIS heterostructures and the influence of interface traps on the capacitance curves shape. By using the basic semiconductor equations we simulated the capacitance–voltage (C–V) curves of these structures for high-frequency signals. We obtained two steps in the C–V curves. The curves with the interface traps are shifted in voltage at both steps depending on whether the trap is of an acceptor or a donor type. The new knowledge found is that the shift caused by the change of the population of interface states is overlapped in almost the whole semiconductor energy gap by a charge change in a two-dimensional electron gas (2DEG). Finally, the presence of deep levels causes widening of the capacitance plateau instead of stretching of the C–V curves. So, the assessment of interface states density for MIS heterostructures is not expected to be so straightforward like in Si metal oxide semiconductor structures.