Abstract: Microalloying of steels refers to adding a small amount of microalloying elements into steels,which can significantly enhance the mechanical properties of steels. In this paper,the research object is medium-carbon microalloyed steel containing vanadium. After aging treatment at 900℃,and isothermal treatment at 680,600,550 and 500℃ respectively,the influence of isothermal temperature on the formation of intragranular ferrite is studied,and the microstructures are observed. The results show that after heat preservation at 900℃,there is a significant increase of ferrite content in the original sample from 5.26% to 37.94%. After isothermal treatment,with the decrease of isothermal temperature,the crystallographic form of ferrite transforms from lumps to short rod-like shape. As the temperature further reduces,it transforms from short rod-like ferrite to acicular ferrite; with the decrease of temperature,ferrite content in the tested steel gradually increases while the grain size gradually decreases.