There exist poor work hardening capacity under medium or low stress condition in conventional Hadfield steels. This poor work hardening capacity together with their low yield strength result in a serious plastic deformation in initial service. To address these two problems, a mechanism had been put forward to explain the unusual work hardening ability of conventional Hadfield steel under heavy stress or high load impact. The formation of deformation twins and its concomitant serious lattice distortion is responsible for their unusual work hardening ability due to the existence of interstitial C atoms. Based on the fact that the same effect can be produced after the formation of stress–induced " martensitic transformation, a high silicon high manganese steel Fe–17Mn–6Si–0.3C was designed. In this alloy the stress–induced " martensitic transformation easily took place under low stress. The mechanical properties and microstructure evolution of the high silicon high manganese steel and a conventional Hadfield steel were studied by OM, XRD and TEM under both static tension and dynamic impact loads. The results showed that under the tension load the high silicon high manganese steel had higher strain hardening rate than the conventional Hadfield steel. Under dynamic impact load the high silicon high manganese steel had lower impact deformation but higher surface hardness than the conventional Hadfield steel. The preferential occurrence of stress–induced " martensitic transformation accounted for this difference. This result also indirectly confirmed that the formation of deformation twins and its concomitant serious lattice distortion due to the existence of interstitial C atoms led to the unusual work hardening ability of conventional Hadfield steel.