The　microstructure　and　properties　of　Fe-1.50　wt%B-0.40　wt%C　high-speed　steel　(i.e.　High　boron　high-speed　steel,　HBHSS)　containing　various　Al　contents　have　been　investigated.　The　results　show　that　the　microstructure　of　HBHSS　is　composed　of　martensite,　a　little　M-6(C,B)　and　a　large　number　of　eutectic　M2B　borides.　With　the　increase　of　aluminum,　the　martensite　can　reduce　and　lots　of　ferrite　and　pearlite　occur,　while　M2B　boride　is　gradually　refined　and　isolated.　Meanwhile,　most　of　aluminum　is　mainly　segregated　within　the　ferrite　grains.　The　room　tensile　strength　of　the　steel　begins　to　decrease　when　Al　content　exceeds　0.6　wt%,　whereas　the　fracture　and　impact　toughness　are　remarkably　enhanced.　A　little　aluminum　can　maintain　high-temperature　tensile　stress　of　the　steel　at　500　degrees　C　and　simultaneously　improve　its　elongation.　The　high-density　dislocations　can　be　formed　within　Al-segregated　ferrite　zone　to　exhibit　the　strengthening　and　toughening　roles,　and　a　possible　orientation　relationship　between　(Fe,Cr)(2)B　and　multi-component　M2B　is　(110)((Fe,Cr)2B)　//　(1.10)(M2B)　in　the　steels.　(C)　2017　Elsevier　B.V.　All　rights　reserved.