This study investigates the effect of rare earth and aluminium composite modification on the structural variations of as-cast and heat treated medium carbon Fe–B cast alloys. The as-cast microstructure of Fe–B cast alloy consists of the eutectic boride, pearlite, martensite and ferrite. Moreover, compared to a netlike distribution of the coarse eutectic borides in the unmodified alloy, the eutectic boride structures in the modified alloy are greatly refined and less interconnected. After heat treatment, the phases in Fe–B cast alloy consist of the boride and martensite. The addition of rare earth helps to increase the number of the rod-shaped and round borides in Fe-B cast alloy during austenitizing. Compared to the unmodified alloy, the boride volume fraction and Rockwell hardness of the modified alloy have no significant change, however, the average area of each boride in the modified alloy is lower and the impact toughness is higher.