The austenite ferrite transformation is the most important reaction route in the manufacture of Fe-based materials. Here the austenite (g) ferrite (a) transformation of pure iron was systematically explored by high-resolution dilatometry. Abnormal transformation ki-netics, multi-peak discontinuous reaction, was recognized in pure iron according to the variation of the ferrite-formation rate. The occurrence the one or the other type of ga trans-formation strongly depends on the grain size: the transfor-mation type changes from abnormal to normal (single-peak continuous reaction) with decreasing grain size. The inherent reason for the occurrence of abnormal transformation could be attributed to the repeated nucleation in front of the mov-ing g/a interface induced by the accumulation of elastic and plastic accommodation energy. The austenite ferrite transformation is the most important reaction route in the manufacture of Fe-based materials. Here the austenite (g) ferrite (a) transformation of pure iron was systematically explored by high-resolution dilatometry. Abnormal transformation ki-netics, multi- peak occurrence discontinuous reaction, was recognized in pure iron according to the variation of the ferrite-formation rate. The occurrence the one or the other type of ga trans-formation strongly depends on the grain size: the transfor-mation type changes from abnormal to normal The single-peak continuous reaction with decreasing grain size. The inherent reason for the occurrence of abnormal phenomenon could be attributed to the repeated nucleation in front of the mov-ing g / a interface induced by the accumulation of elastic and plastic accommodation energy.