In　this　work,　the　effect　of　solidification　rate　on　the　microstructure　and　impact　toughness　of　high　boron　high　speed　steel　is　systematically　investigated.　The　theoretical　and　experimental　results　indicate　that　the　as-cast　microstructure　of　high　boron　high　speed　steel　consists　of　alpha-Fe　matrix　and　eutectic　borocarbide.　Without　Ca-Ti　addition,　borocarbide　presents　a　continuous　and　reticular　structure.　After　Ca-Ti　modification,　borocarbide　possesses　an　isolated,　fine　structure.　Besides,　cooling　rate　obviously　affects　the　extent　of　spheroidization.　In　modified　high　boron　high　speed　steel,　calcium　possesses　surface　activity,　which　restrains　the　growth　of　eutectic　borocarbide　during　solidification.　Titanium　exists　in　the　form　of　dispersedly　distributed　TiC　particles　which　act　as　heterogeneous　nuclei　for　eutectic　borocarbide.　The　impact　toughness　of　high　boron　high　speed　steel　is　remarkably　improved　by　Ca-Ti　modification,　which　can　be　further　improved　by　cooling　rate.　The　effect　of　cooling　rate　on　improving　of　the　toughness　can　provide　a　scientific　basis　for　the　foundry　process　designing.