Abstract: The water quenched low-carbon Si-Mn steel subsequenty annealed in the two-phase region and isothermal treated in the bainite region.Optical microscopy,scanning electron microscopy and transmission electron microscopy were applied to observe the microstructure of the steel after heat treatment,X-ray diffraction analysis was used to determined the residual austenite content in the steel.The tensile test was performed to deterime the mechanical properties of the steel.The results show that after annealing in two-phase region the microstructure of the steel was ferrite and martensite.With the increase of annealing temperature and holding time,the ferrite content was reduced and martensite content increased.Most of the ferrite were bar-like.After isothermal treatment in the bainite region,the majority of lath-shaped residual austenite existed at the lath bounderies of bainite,farthing existed in the massive proeutectoid ferrite body,and its content increased first and then decrtased with increasing the annealing temperature and holding time,the the maximum content appeared at 780℃ for 5 min.The tensile strength and yield strength of the steel monotenically increased with increasing the annealing temperature and holding time and the elongation reached a maximum value at 780 ℃ for 5 min.