Abstract: The continuous cooling transformation (CCT) of low carbon Mn-Nb steels with/without boron was investigated by thermal-mechanical simulator. The CCT curves of two kinds of steel were obtained and the microstructures and microhardness of the samples with different cooling rate were measured. The results showed that deformation moves the CCT curves to the left, increases the transformation start temperature, and promotes the formation of polygonal ferrite and pearlite. A trace boron addition moves the CCT curves to the right, decreases remarkably the transformation start temperature, suppresses obviously the formation of polygonal ferrite and pearlite, decreases the cooling rate range of polygonal ferrite and pearlite, and increases effectively the microhardness of low carbon Mn-Nb steel.