Abstract: The microstructures and mechanical properties of a low carbon Mn-Nb-B bainitic steel were investigated. The results revealed that when the finish rolling temperature is 850°C, the cooling rate is about 10°C/s and the stop cooling temperature is 560°C. The microstructure of tested steel is quasi-polygonal ferrite and granular bainite. The yield strength, tensile strength, impact energy (-20°C) and elongation are 495 and 720 MPa, 159 J, 23%, respectively. As the stop cooling temperature is decreased to 480°C, the yield strength and tensile strength are increased 51 MPa and 93 J respectively. The yield strength is increased 24 MPa when the finish rolling temperature is decreased to 810°C. When the cooling rate is increased to 25°C/s, the microstructure is composed of granular bainite, acicular ferrite and lath bainite. The yield strength, tensile strength, impact energy (-20°C), elongation are 655 and 777 MPa, 226 J, 20%, respectively. The substructure can be refined by accelerated cooling. The strength of tested steel can be increased observably through the substructure strengthing and the existence of lath bainite.