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($2{ imes}10^{-3}/sec$ and $2{ imes}10^{-1}/sec$). However, the true stress increased at all strain levels when the strain rate increased to $6{ imes}10^3/sec$. Based on the experimental results, an constitution equation to calculate the dynamic strength for strain rates over $10^4/sec$ was determined. The Fe-5%Mn alloy containing athermal ${alpha}^{prime}$ 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 ${varepsilon}$ martensite but to the strain induced transformation (${gamma}{ ightarrow}{varepsilon}$ and ${varepsilon}{ ightarrow}{alpha}^{prime}$) occurring during each stages of deformation.