This　study　investigates　the　effect　of　cerium　on　the　microstructures,　mechanical　properties　of　medium　carbon　Fe-B　cast　alloy.　The　as-cast　microstructure　of　Fe-B　cast　alloy　consists　of　the　eutectic　boride,　pearlite,　and　ferrite.　Compared　with　the　coarse　eutectic　borides　in　the　unmodified　alloy,　the　eutectic　boride　structures　in　the　modified　alloy　are　greatly　refined.　Cerium　promotes　the　formation　of　Ce2O3　phase.　Ce2O3　can　act　as　effective　heterogeneous　nuclei　of　primary　austenite,　and　refine　austenite　and　boride.　After　heat　treatment,　the　impact　toughness　of　the　modified　alloy　is　higher　than　that　of　the　unmodified　alloy　because　there　are　more　broken　borides　in　the　modified　alloy.　Meanwhile,　the　fracture　mechanism　of　medium　carbon　Fe-B　alloy　is　depicted　and　analyzed　by　using　fractography.