Bainite in microalloyed steel possesses excellent synthetic mechanical properties. However, it will probably evolve towards equilibrium microstructure when it is subjected to thermal disturbance. In addition, bainite frequently undergoes more or less deformation during manufacture of steel structure. In the present investigation, cold deformation test, isothermal heat treatment, hardness measurement, optical microscopy and transmission electron microscopy were employed to detect evolution behavior of bainite isothermally held below A1 temperature. It was found that hardness of samples drops in generally during the isothermal heat treatment. Meanwhile, bainite evolves gradually into polygonal ferrite. Cold deformation enhances the initial hardness of samples while it largely accelerates softening and evolution of microstructure towards equilibrium one during heating. Almost same effects are produced by tensile strain and compressive strain. Higher solubility product of carbon and niobium results in higher thermal stability of bainite. Cold deformation cause dislocations in bainitic laths to distribute heterogeneously and most dislocations pile up along boundaries. During isothermal holding, dislocations redistribute further followed by extending of low dislocation density areas across lath boundaries. Finally, polygonal ferrite nucleates in those areas and grows gradually.