An amorphous SiO2/4H–SiC (0001) interface model with carbon dimer defects is established based on density func-tional theory of the first-principle plane wave pseudopotential method. The structures of carbon dimer defects after pas-sivation by H2 and NO molecules are established, and the interface states before and after passivation are calculated by the Heyd–Scuseria–zerhof (HSE06) hybrid functional scheme. Calculation results indicate that H2 can be adsorbed on the O2–C=C–O2 defect and the carbon–carbon double bond is converted into a single bond. However, H2 cannot be ad-sorbed on the O2–(C=C)’–O2 defect. The NO molecules can be bonded by N and C atoms to transform the carbon–carbon double bonds, thereby passivating the two defects. This study shows that the mechanism for the passivation of SiO2/4H–SiC (0001) interface carbon dimer defects is to convert the carbon–carbon double bonds into carbon dimers. Moreover, some intermediate structures that can be introduced into the interface state in the band gap should be avoided.