It was found that more than 60% of the steam turbine blade damages occurred under the condition alternatively repeated wet and dry of vapor and condensed vapor at the lower pressure stage. And also, it has been well known that both the mechanical properties and environmental strength of the steam turbine blade can be changed by the crystal structure. However, in spite of these common facts, it is difficult to find out the quantitative results including the particular environmental condition as well as the actual service conditions. In this study, as a fundamental investigation to provide design information and reliability evaluation of the 12Cr alloy steel used for a steam turbine blade, stress corrosion strength of the 12Cr alloy steel of which its crystal structure is different was assessed under $2.5{sim}3.5wt.%$ NaCl solution at 90oC. From the results, S-t curves for predicting damage life and design criterion of the 12Cr alloy steel including corrosion environment as well as S.C.C. condition were obtained.