Abstract: The effects of cooling rate on both austenite grain size and ferrite grain size at room temperature before ferrite transformation were investigated on MMS-300 thermomechanical simulator with the specimens of a kind of C-Mn steel to which the trial rolling processes were done simultaneously on rolling mills with conventional low-temperature controlling and non-low temperature controlling plus forepart fast cooling technologies. Some results revealed that: (1) the austenite grain size decreases with the increasing cooling rate before ferrite transformation; (2)the ferrite grain size at room temperature becomes finer with the increasing cooling rate before ferrite transformation; (3)the excellent mechanical properties provided by the technology of non-low temperature controlling rolling plus forepart fast cooling are the same to that by the low-temperature rolling technology, but the former has lower yield/tensile ratio.