Microalloying effects of sulfur are investigated systematically on the steam oxidation resistance of austenitic steels and ferritic steels. It is found that the presence of S, about 50ppm, in the steels improves remarkably the steam oxidation resistance of both austenitic steels and high Cr ferritic steels. This result indicates that ferritic steels with excellent steam oxidation resistance can be designed and developed by just leaving S at an impurity level in them without detriment to the mechanical strength. However, the S effect is not so remarkable in low Cr ferritic steels as in high Cr ferritic steels. This fact implies that the S effect is related strongly to the stability of Cr2O3 in high temperature steam conditions.