Abstract: The effects of two different thermomechanical control processes (TMCP) including medium temperature slow cooling and medium temperature isothermal treatment after roiling on the microstructure and mechanical properties of Si-Mn series bainitic steel were investigated in this paper. The mechanical properties of the steel were studied by a tensile testing machine, the microstructure was characterized by means of scanning electron microscopy and electron backscatter diffraction, and the amount of retained austenite in the steel was determined by X-ray diffraction analysis. The results show that, as the starting slow cooling temperature after rolling rises, the tensile strength, hardness and tensile strain hardening exponent n increase, the elongation rate and the impact toughness decrease, and the yield-strength ratio decreases first and then increases. When the isothermal time after rolling extends, the tensile strength and the yield-strength ratio of the steel first decrease and then increase, but the elongation and the impact toughness have a reverse trend. Compared with the isothermal regime, the continuous slow cooling can get better comprehensive mechanical properties, the product of tensile strength and elongation is higher, and the elongation is 20% higher than that of the former.