Abstract: Studied is the major element among those concerning control of the toughening orientation and toughness of the low-carbon Mn-Mo-Nb-B series steels, i.e., the control of the primitive gains.The result shows that with heating temperature of about 1 250℃ and holding time of 5 min/mm, the austenite grain size still remains as about 50 μm, a very appropriate grain size for austenitic recrystallisation-rolling, and that nevertheless, in case of temperature above 1 250℃, the grain size shall become bigger rapidly.This conclusion can be used as reference basis for establishing more logic heating schedule for steels of the above mentioned category.