In martensitic steels, it is well known that a certain chemical driving force (about 180 MJ/m³) is required to start martensitic transformation (Ms), and additional driving force has to be charged further to complete the transformation (Mf). In the case of metastable austenitic steels with Ms temperature at around room temperature, however, only the chemical driving force needed to start martensitic transformation has been stored at room temperature. Hence, the state of austenite is very unstable thermally. It has already been known that such a metastable austenite undergoes a partial martensitic transformation during isothermal holding at room temperature or cooling to a low temperature. It is very convenient to investigate the behavior of martensitic transformation of austenite. In this study, the effect of austenite grain size on martensitic transformation is introduced from the viewpoint of microstructural analysis and thermo-dynamics. The steel used in this investigation is an Fe-16 mass%Cr-10 mass%Ni ternary alloy, which has Ms temperature at around room temperature. The grain size of this steel can be controlled from 0.8 μm to 80 μm using the technique of reversion of deformation induced martensite. In the material with coarse grain size (80 μm), about 18% of martensite was detected at room temperature and the amount of martensite was increased to 50% by the following subzero treatment to 77 K. However, martensite was hardly detected in the material with ultra fine grains (0.8 μm) even after the subzero treatment. It was found that such a stabilization occurs in the materials with the grain size below 10 μm and the stabilization was reasonably explained by considering the relation between austenite grain size and elastic strain energy which is required on the single variant martensitic transformation.