A calculation based on classic diffusion controlled nucleation and ledgewise growth theory is conducted in this study to analyze the sympathetic nucleation at growth ledge terraces of the bainitic ferrite plate in steel. The competition between sympathetic nucleation and the ledgewise growth is investigated quantitatively. The rationality of sympathetic nucleation on the ledge terraces of a previous bainitic ferrite in competition with lateral growth of ledge risers is demonstrated in this work. Low reaction temperature and high carbon concentration promote sympathetic nucleation more than growth of ledge risers. Due to increasing sympathetic nucleation, a bainitic ferrite with ultra-fine inner structures (that is the case for lower bainite) is preferred at low temperatures and high carbon alloys. On the contrary, high reaction temperature and low bulk carbon concentration lead to the formation of the bainitic ferrite with less inner structures (that is the case for upper bainite).