Abstract: The effect of isothermal treatment at temperatures in the range of 700, 650, 600 and 550℃, and that of isothermal holding time at 600℃ on formation of intra-granular ferrite (IGF) in Mg treated steel were studied by using thermal simulation.Vickers hardness test was also carried out for the steel isothermally treated at different temperatures, and the constituent of the inclusions that enhance the formation of intragranular ferrite was analyzed by EDAX.The results show that the amount of IGF reach to maximum at 650℃and the hardness of the steel decreases at first and then increases, when the isothermal temperature decreases from 700℃ to 550℃.However, intra-granular ferrite disappears when the isothermal temperature decreases to 550℃.It is found that IGF and bainite competed to form in the austenite grains at 600℃.The further analysis indicates that inclusions consisting of Mg, Al and Ti could induce the IGF nucleation.