Dilatometric experiment and thermodynamic calculation have been performed to determine M_s, A_s and driving forces for γ→ε 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 γ→ε 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 ΔT(50K∼80K) for γ→ε martensitic transformation indicated that Fe-Mn alloys behave like thermoelastic martensitic alloys : We would like to call them semi-thermoelastic martensitic alloys.