Effect of sustenite grain size on starting temperature of γ→ε martensitic transformation(M_s) has been studied in an Fe-18%Mn alloy. Particular attention was paid on the variation of stacking fault energy with austenite grain size, which is considered to be a important factor affecting γ→ε martensitic transformation. Austenite grain size was increased in a wide range from 13,um to 185 with increasing solution treatment temperature from 700℃ to 1100℃. Hardness was decreased with increasing austenite grain size while the volume fraction of a martensite showed a reverse tendency, which indicates that the hardness is more dependent on austenite grain size thane martensite content. No significant change was found in M_s temperature when the grain size was larger than about 30㎛. In case that, the austenite grain size was smaller than about 30㎛, however, M_s temperature was marlkedly decreased with decreasing austenite grain size. A linear relationship between M_s temperature and the stacking fault formation probability, i.e. the reciprocal of the stacking fault energy was obtained, which suggests that the variation of M_s temperature with austenite grain size is closely related to the change in stacking fault energy.