We proposed an electron cyclotron resonance microwave hydrogen-nitrogen mixed plasma (HNP) pretreatment for 4H-SiC surface combined with post-oxidation annealing (POA) to improve the SiO2/SiC interface properties. Results revealed that HNP surface pretreatment effectively reduced the density of interface traps (D-it), which was closely correlated with interface flattening because of surface flattening, surface state (contaminants, adsorbates, and dangling bonds) reduction, and suppressed generation of interface defects during oxidation. Combined with POA, D-it was further decreased because of passivation of the formed defects after oxidation. The correlation among passivation, SiC surface properties, SiO2/SiC interface properties, and defect levels was established. (C) 2015 Elsevier B.V. All rights reserved.