A new explanation of the physical significance of the B 0 (the theoretical upper temperature limit) and the B S (the practical upper temperature limit) temperatures in steel is proposed. It is found that the B 0 temperature of the bainite reaction in steel should be defined by the point of intersection between the thermodynamic equilibrium curve of austenite->ferrite transformation by coherent growth (curve Z γ - >) and the thermodynamic equilibrium curve of austenite->cementite transformation (curve ES in the Fe-C phase diagram). The B S temperature for the bainite reaction is approximately 50~55 o C lower than the B 0 temperature. Using this method, the B S and B 0 temperatures for plain carbon steel were found to be 630 o C and 680 o C, respectively. The effects of steel composition on the B S temperature are discussed. The formula B S ( o C)=630-45 Mn-40 V-35 Si-30 Cr-25 Mo-20 Ni-15 W can be used to predict the B S temperature of a wide range of commercial steels.