Abstract: Microstructure of low carbon high niobium steel under the conditions of different controlling rolling and cooling parameters was studied by thermomechanical simulator. The results show that the amount of ferrite increased and the amount of bainite decreased with the decrease of temperature and the increase of deformation, and the morphology of the lathe bainite became granular gradually. When the cooling rate was lower than 5 ℃·s-1, the microstructure was constituted by ferrite and pearlite, and when the cooling rate was higher than 5 ℃·s-1, the amount of bainite increased and the grains were smaller and more uniform. The micro-hardness of tested steel increased with the increase of cooling rate.