We performed high-pressure H2O vapor annealing on 4H-SiC n-MOS capacitors to control SiO2/4H-SiC interface properties. High-pressure H2O annealing was performed at 270～ 420oC at a pressure of 1.31～1.67MPa. Effective negative fixed oxide charge decreased with increasing anneal temperature in the case annealed with Al gate electrodes. However, it increased with increasing anneal temperature in the case annealed without Al gate electrodes. The effect of annealing was much larger on the C-face than that of Si-face. Interface state densities near the conduction band edge was decreased at 420oC under 1.67MPa compared to other samples, especially on the C-face n-MOS capacitors.