The microstructure of Mn-Mo-Nb-B microalloyed steel was investigated under different cooling rates after austenitic deformation by using thermo-simulation test, hardness measurement, metallographic and TEM analysis. The experimental results show that most constituents are mixed bainite ferrite and granular bainite at the cooling rate of 10 oC-50 oC /s. From the CCT curves, only polygonal ferrite and martensite could be obtained through the very slow and rapid cooling, respectively. The hardness of specimens keeps almost unchanged when cooling rate reaches above 10 oC /s (the utmost cooling rate is 50 oC /s in this simulation). Hence, the microstructure uniformity is attained, that is to say, the transformation products are insensitive to the cooling rates, which attributes to the appropriate component design and optimized hot simulation. Another experimental phenomenon is that the fine scale (<5 nm) carbonitride precipitates were observed in the relatively lower cooling specimens.