Abstract: The continuous cooling and isothermal transformation microstructure evolution in a low carbon Mo-Cu-Nb-B microalloying steel were investigated. It was found that acicular ferrite could be transformed independently in the intermediate temperature range of metastable austenite. Although multi intermediate transformation phases can be transformed during the continuous cooling, the fraction of phases, especially acicular ferrite, cannot be controlled. However, the fraction of acicular ferrite could be controlled by the step cooling process. The optimum amount of acicular ferrite divides the prior-austenite grains, resulting in multi-phase mierostrueture ultrafine.