In this paper, we have reported high pressure oxidized thin aluminium layer as a gate dielectric for GaN-based MIS-HEMTs and studied the interface traps at Al2O3/III-Nitride interface using the capacitance-conductance method. Effect of oxygen plasma treatment prior to aluminium layer deposition has also been investigated. Significant reduction in gate leakage current has been observed in all fabricated MIS-HEMTs compared to reference HEMT in both reverse and forward bias conditions. Forward bias swing is also larger for MIS-HEMTs. Significant reduction in interface trap density was found for MIS-HEMTs with oxygen plasma treatment. © 2018 IEEE.