The　equilibrium　solidified　phase　diagrams　of　high　boron　high　speed　steels　with　different　content　of　aluminum　were　calculated　and　the　vertical　section　of　the　Fe-B　pseudo-binary　phase　diagrams　were　drawn　by　therm-Calc　software.　The　effect　of　aluminum　content　on　the　phase　diagrams　and　solidification　microstructure　was　investigated　by　OM,　SEM,　XRD　and　DSC.　The　results　show　that　the　eutectic　transition　temperature　and　eutectic　point　have　no　significant　change　with　the　increase　of　aluminum　content,　but　the　austenite　region　is　shrank　sharply.　When　the　aluminum　content　is　1.5　mass%,　the　austenite　region　is　reduced　to　a　closed　circle　and　the　δ-Fe　changes　into　α-Fe　directly　in　cooling　process.　The　lath　martensite　in　the　as-cast　structure　for　the　alloy　with　aluminum　content　of　1.0%　is　transformed　into　pearlite　for　the　alloy　with　1.5%Al.　The　addition　of　excessive　amount　of　aluminum　favors　the　formation　of　ferrite,　which　leads　to　the　hardness　decreasing.　Moreover,　the　excessive　amount　of　aluminum　(1.5　mass%　Al)　makes　network　borocarbides　tend　to　break.　The　calculation　results　are　agreed　with　the　ones　from　the　experiment.　©　2016,　Editorial　Office　of　Transactions　of　Materials　and　Heat　Treatment.　All　right　reserved.