We investigated the effect of boron passivation and re-oxidation on the properties of the silicon carbide/silicon dioxide interface. Metal-oxide-semiconductor capacitors were fabricated on 4H-silicon carbide substrates and the capacitance-voltage properties were measured. The high-low capacitance-voltage method was used to obtain the interface trap density from the capacitance-voltage curve. Boron passivation is known to be effective in reducing the size of carbon clusters at the silicon-carbide/silicon-dioxide interface. Also re-oxidation is known to be effective in improving the quality of the oxide and reducing the dangling bond set of the silicon-carbide/silicon-dioxide interface. The effect of each boron passivation and re-oxidation method on the silicon carbide/silicon dioxide interface was analyzed by observing the interface trap density obtained from the capacitance-voltage curves. We found that the interface trap density could be significantly improved; the best sample exhibited an interface trap density approximately 51% lower than that of the sample subjected to conventional oxidation via wet oxidation, boron passivation and wet re-oxidation. The interface of each sample was investigated with X-ray photoelectron spectroscopy, based on which we inferred that boron-passivation reduced the size of residue carbon clusters located at the silicon-carbide/silicon-dioxide interface.