The high strain-rate dynamic plastic behavior of Fe-X%Mn alloys was investigated. The strain rate did not have an effect when tested under quasi-static strain rates(2×10^(-3)/sec and 2×10^(-1)/sec). However, the true stress increased at all strain levels when the strain rate increased to 6×10³/sec. Based on the experimental results, an constitution equation to calculate the dynamic strength for strain rates over 10⁴/sec was determined. The Fe-5%Mn alloy containing athermal α` martensite initially did not show work hardening. The work hardening increased with Mn content showing a maximum at 20% Mn. The high work hardening of Fe-20%Mn and Fe-30%Mn alloys appears to be closely related not only to the initial amounts of E martensite but to the strain induced transformation (γ→ε and ε→α`) occurring during each stages of deformation.