Abstract: Optical microscopy(OM) and scanning electron microscopy(SEM) were used to observe the microstructure of ultra low carbon bainite steel(ULCB) that experienced controlled rolling and controlled cooling.Results show that it is more appropriate for SEM to reveal the characteristics of microstructure of the ULCB.The longitudinal microstructure of ULCB is fine and austenite grain is flat along the rolling direction and austenite grain boundary is very clear,the width of austenite grain is about 6-13 μm.Along the transverse direction,granular bainite is dominant at the center and surface of the sample,grain is coarse,equiaxed and even.There are distinct differences between granular bainite and lath bainite at the two extremity temperature and it is very difficult to distinguish them at the transitional temperature.