Effect of high-pressure water vapor (HPV) annealing is discussed from density of state (DOS) by capacitance–voltage (C–V) method and ΔV FB by the cyclic C–V measurement. The DOS of HPV samples were smaller than that of conventional atmosphere (AT) annealing around conduction band minimum (E c). The ΔV FB of HPV samples were also smaller than that of AT. This suggests that HPV annealing is an effective method to decrease electron trap density as compared with AT condition. Especially, HPV 0.5 MPa sample was lower electron trap density and more stable than the other pressure HPV samples. Therefore, it is considered that the HPV in 0.5 MPa is the most promising condition. In addition, we succeeded in demonstrating the analysis of trap density in thin film by C–V method and cyclic C–V measurement.