Abstract: The effect of C and Mn contents on ferrite nucleation rate during Deformation-Enhanced Ferrite Transformation (DEFT) in low carbon (Mn) steels was investigated. The results show that the deformation causes such a higher defect density in austenite grains, which leads to extra sites for nucleation both at the austenite boundaries and intragranular, that ferrite grains are refined dramatically. The addition of C or Mn is favored to strain cumulated in the austenite, which also increases the nucleation sites, resulting in more pronounced ferrite grains refined. The promotive ability of C to refine ferrite grains is, however, much greater than that of Mn.