The microstructure and mechanical properties of NANOBAIN steel treated at different isothermal temperatures were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), uniaxial tensile tests and X-ray diffraction (XRD). It was found that bainitic ferrite (BF) plate was made of basic shear transformation units arranged in the same direction of subunits. The existence of defects, such as nanoscale twinning and dislocation, suggested that the growth of transformation units was controlled by the surrounding defect plane with dislocation, which was consistent with the moving direction of BF/austenite interface parallel to the twinning plane. The behavior of work hardening indicated that mechanical stability of microstructures obtained at 250 ℃ and 300 ℃ was much more stable than that obtained at 210 ℃. The evolution of carbon partitioning in retained austenite and bainitic ferrite also indicated that austenite was enriched in carbon at the initial stage step by step; after the formation of BF, the austenite did not seem to be greatly enriched in carbon and the carbon content showed a little decrease instead; subsequently, a phenomenon of little decarburization of supersaturated bainitic ferrite has also been found.