Abstract: After hot deformation and relaxation of austenite,the kinetics,especially the microstructure evolvements of low carbon bainitic steel were studied with Thermecmastor Z simulator.With the increasing of strain,Bs and the maximum bainitic volume fraction decreases,however,transformation rates at the initial stage are accelerated.With the increasing of the relaxation time after hot deformation,Bs and the volume fractions of bainite transformation increase,and approach to a maximum value.The relaxation makes the boundaries of bainitic plates become clear and the individual plate becomes thinner.