The microstructure of bainite ferrite in NANOBAIN steel transformed at diff erent temperatures was investi-gated by scanning electron microscopy, transmission electron microscopy, electron back-scattered diff raction, and vickers hardness tester in detail. It is found that the average width of bainitic ferrite (BF) plates can be refined to be thinner with the reduction of temperature (473-573 K), and the bainitic ferrite plates can reach up to 20-74 nm at 473 K. Crys-tallographic analysis reveals that the bainitic ferrite laths are close to the Nishiyama-Wasserman orientation relationship with their parent austenite. Temperature shows a significant eff ect on the variant selection, and a decrease in temperature generally weakens the variant selection. Thermodynamic analyses indicates that the Lacher, Fowler and Guggenheim (LFG) model is more suitable than the Kaufman, Radcliff e and Cohen (KRC) model dealing with NANOBAIN steel at a low temperature range. The free energy changeΔGγ →BF is about-1500 J·mol-1 at 473 K, which indicates that nucleation in NANOBAIN steel is the shear mechanism. Finally, the formation of carbon poor regions is thermodynamically possible, and the existence of carbon poor regions can greatly increase the possibility of the shear mechanism.