The reactions of NO molecule at 4H-SiC/SiO2 interface on various plane orientations are theoretically investigated using density functional calculations to clarify microscopic mechanisms of NO post oxidation annealing (POA) process. We find that the energy barriers for nitrogen incorporation reaction are smaller than those for nitrogen desorption irrespective of the plane orientation, indicating that N atoms are preferentially incorporated. However, on the Si-face we find a possible NO reaction process without CO molecule formation as well as the reaction with NCO molecule formation when the interface includes pre-incorporated N atoms. Owing to the reaction resulting in the formation of NCO molecule, the incorporation of N atoms can be suppressed, which could be one of possible origins for the N density saturation in the NO-POA. The calculated results suggest that not only structural stability of the interface but also reaction kinetics are crucial for the incorporation of N atoms during the NO-POA.