Alloy-designing of high-strength ferritic heat resistant steels has been investigated for application to highly efficient, low emission ultra-supercritical power plant with maximum steam temperature of 700 oC. Ferritic heat resistant steels can be applied to the next highest temperature components and are strongly desired to expand their temperature range up to 650 oC in order to minimize the requirement of expensive nickel base superalloys. A dispersion of nanometer size MX nitrides along boundaries and the addition of boron significantly improve long-term creep strength. Newly alloy-designed 9Cr-3W-3Co-0.2V-0.05Nb steel with 160 ppm boron and 85 ppm nitrogen exhibits excellent creep strength of base metal and no degradation in welded joints at 650 oC. The protective Cr2O3-rich scale forms on the surface of 9Cr steel by pre-oxidation treatment in Ar gas, which significantly improves the oxidation resistance in steam at 650 oC.