With an in situ low-damage NH3-Ar-N2 plasma pre-gate treatment, a high-quality Al2O3/GaN-cap interface has been obtained in the Al2O3/GaN/AlGaN/GaN MIS-structures. Frequency- and temperature-dependent C-V characterization techniques were developed to map the interface trap density (Dit) at the dielectric/III-nitride interface, whereby a low Dit of ∼10 12-1013 cm-2eV-1 in the Al 2O3/GaN/AlGaN/GaN MIS-structures was extracted. The mechanism for the high-quality interface was validated to be effective removal of native oxide and the subsequent formation of a monocrystal-like nitridation inter-layer on the GaN surface. Both Dit mapping and the pre-gate treatment techniques are of significance for the improvement of III-nitride MIS-HEMTs. © 2013 IEEE.