Dilatometric experiment and thermodynamic calculation have been performed to determine $M_s$, $A_s$ and driving forces for ${gamma}{ ightarrow}{varepsilon}$ martensitic transformation of Fe-Mn alloys. The transformation temperatures($M_s$, $A_s$, $T_o) were decreased with increasing manganese content and were newly formulated as a function of manganese content. Driving force for ${gamma}{ ightarrow}{varepsilon}$ martensitic transformation was increased from -75J/mole to -105J/mole with increasing manganese content from 15wt.% to 25wt.%. Transformation temperature hysteresis($A_s-M_s$) was also increased from 50K to 80K with increasing mangenese content from 15wt.% to 25wt.%. The small driving force(-75J/mole~-105J/mole) and small ${Delta}T$(50K~80K) for ${gamma}{ ightarrow}{varepsilon}$ martensitic transformation indicated that Fe-Mn alloys behave like thermoelastic martensitic alloys : We would like to call them semi-thermoelastic martensitic alloys.