The effect of ausforming on the bainitic transformation velocity and transformed volume fraction in a low carbon boron steel is twofold. They increase with ausforming strain at small strain range, but decrease with strain at large strain range. On the contrary, for the same steel without B addition, both transformation velocity and transformed volume fraction always decrease with auforming strain. For the steel with B addition, ausforming can reduce the segregation of B at austenite grain boundaries as some B atoms segregate at dislocations induced by deformation, resulting in the increase of bainite nucleation rate. Furthermore, auforming can refine the volume of a single bainite sheaf. Based on the competing mechanisms between increase of nucleation rate and the refinement of bainite sheaf structure, the effects of B and ausforming on the bainitic transformation are discussed.