In this study, conventional multiple-frequency capacitance-voltage (C-V) curves of a metal-insulator-semiconductor structure were modified into barrier capacitance-barrier voltage (Cbr-Vbr) curves to study the interface state of an Al2O3/III-N interface. By eliminating the contribution of a serially connected insulator capacitance (Ci), the variation of Cbr as a function of Vbr could be obtained for determining the interface-state-caused dispersion precisely. This technique was used to deduce the distribution of energy-level-dependent Dit and to investigate the influence of thermal oxidization on an Al2O3/AlGaN/GaN heterostructure with an in situ grown AlN/GaN cap layer. The oxidization process was performed prior to Al2O3 deposition. Depositing an Al2O3 layer on an oxidized wafer by using an atomic layer deposition system can considerably reduce the dispersion behavior among frequencydependent Cbr-Vbr curves. By determining the amount of frequency dispersion among the Cbr-Vbr curves, the distribution of Dit was determined to be in the range between 1.4 ×1012 and 2.6 ×1013 eV-1 ×cm-2, which was an approximately one-order-of-magnitude reduction compared with a structure that was not subjected to the oxidization process.