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.