Chromium is an effective element for improvement in the oxidation resistance of steels. However, chromium addition to ferritic creep resistant steel is restricted to around 9-10 wt.%, since excess addition of chromium results in formation of delta-ferrite which decreases creep strength and toughness. In this study, influence of normalizing heat treatment under high pressure on microstructure and creep strength has been investigated in a steel containing 15 wt.% of chromium, since austenite single phase region is extended up to about 20 wt.% of chromium under high pressure of about 4 GPa. In contrast to a ferritic microstructure of the steel normalized under atmospheric pressure of 100 kPa, the martensitic one was found in steel normalized under high pressure of 4 GPa. Hardness in the as tempered condition of the steels subjected to normalizing heat treatment at 1273 and 1173 K under 4 GPa was 290 and 289 HV, respectively. This is significantly higher than the value of 182 HV for the steel normalized at 1273 K under atmospheric pressure of 100 kPa. Creep rate of the steel normalized under high pressure of 4 GPa was significantly smaller than that of the steel normalized under pressure of 100 kPa. It has been concluded that high pressure normalizing heat treatment is effective to suppress a formation of delta-ferrite and to improve both creep strength and oxidation resistance.