The atomic structures of the 4H-SiC/SiO₂ interface for the a, m, C, and Si faces after NO annealing are investigated by density functional theory calculation. The nitride structures preferentially form at the topmost layers of the SiC side of the interface, which agrees with the experimental finding reporting that the N atoms accumulate at the interface. The electronic structure of the interface after NO annealing where the nitride layer is inserted, has no gap states. It is fair to conclude that NO annealing can contribute to the reduction in the density of interface defects by forming the nitride layer.