Dry-oxidized and N2O-annealed n-type 6H-SiC metal-oxide-semiconductor capacitors are investigated at room temperature using a high-frequency C-V technique. Lower oxide-charge and interface-state densities are observed in case of dry-oxidized device. Oxide reliability of the capacitors is also investigated under high-field stress. Compared to a dry-oxidized device, smaller change in interface-state density occurs for a nitrided device under high-field stress. On the other hand, positive flat-band shift of nitrided device is found to be larger after the stress, implying that the nitridation creates acceptor-type interface states and oxide traps in the device. In summary, N2O nitridation improves the SiO2/n-type 6H-SiC interface and oxide hardness against stress-induced damage.