Effect of austenite grain size on starting temperature of ${gamma}{ ightarrow}{varepsilon}$ 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 ${gamma}{ ightarrow}{varepsilon}$ martensitic transformation. Austenite grain size was increased in a wide range from $13{mu}m$ to $185{mu}m$ with increasing solution treatment temperature from $700^{circ}C$ to $1100^{circ}C$. Hardness was decreased with increasing austenite grain size while the volume fraction of ${varepsilon}$ martensite showed a reverse tendency, which indicates that the hardness is more dependent on austenite grain size than ${varepsilon}$ martensite content. No significant change was found in $M_s$ temperature when the grain size was larger than about $30{mu}m$. In case that, the austenite grain size was smaller than about $30{mu}m$, 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.