The carbon and nitrogen free new alloys which were composed of the supersaturated martensitic microstructure with high dislocation density before the creep test have been investigated systematically. These alloys were produced from the new approach which raised creep strength by the utilization of the reverse transformed austenite phase as a matrix and intermetallic compounds such as Laves and μ-phases as precipitates during creep test. It is important that these alloys are independent of any carbides and carbo-nitrides as strengthening factors. Creep behavior of the alloys is found to be different from that of the conventional high-Cr ferritic heat resistant steels. The minimum creep rates of the Fe-Ni alloys at 700°C are found to be much lower than that of the conventional steel, which is due to fine dispersion strengthening useful even at 700°C in these alloys. As a result carbon and nitrogen free alloys exhibited superior creep properties at temperatures more over 700°C, and steam oxidation resistance.