The　microstructure　and　crystallography　of　eutectic　borides　and　secondary　precipitations　in　18　wt.%　Cr-4　wt.%　Ni-1　wt.%　Mo-3.5　wt.%　B-0.27　wt.%　C　steel　have　been　investigated　extensively.　The　results　show　that　the　as-cast　microstructure　of　Cr-Ni-Mo-containing　Fe　B　steel　is　composed　of　a　dendritic　martensite　with　large　interdendritic　eutectic　borides.　Transmission　electron　microscopy　(TEM)　results　confirm　that　the　borides　are　indexed　to　Cr-　and　Mo-rich　M2B-type　borides　with　the　chemical　formulas　of　Fe(1.35-1.36)Cr(0.92-1.05)B0.96　and　Fe0.73Cr0.45Mo0.78B,　respectively.　The　cluster-like　boride　possesses　a　possible　orientation　relationship　between　body-centred　orthorhombic　Cr-rich　M2B　and　martensite　with　＜　1　(1)　over　bar　0　＞(M2B)//＜　1　1　0　＞(alpha)　growth　direction.　After　destabilization,　M-23(C,　B)(6)　secondary　borocarbide　with　a　specific　orientation　relationship　precipitates　first　and　thereafter　coarsens　following　the　appearance　of　M-7(C,　B)(3)　precipitation　with　the　increasing　destabilization　temperature　at　the　same　soaking　time,　thus　leading　to　a　large　decrease　of　Cr　content　in　the　martensite.　However,　no　M-6(C,　B)　secondary　borocarbide　is　found　in　as-destabilized　Fe　B　steel.　Destabilization　treatment　has　no　effect　on　the　morphology　of　eutectic　borides.　The　secondary　borocarbides　have　the　stoichiometry　of　(Fe18.26Cr4.74)(B,　C)(6)　and　(Fe3.86Cr3.14)(B,　C)(3)　respectively.　The　high-resolution　TEM　results　indicate　that　the　nucleation　and　precipitation　of　M-23(C,　B)(6)　occur　at　the　grain/sub-grain　boundaries　as　well　as　partial　within　martensite,　and　a　subsequent　transformation　from　M-23(C,　B)(6)　to　M-7(C,　B)(3)　takes　place　in　situ,　which　is　probably　owing　to　the　crystalline　defects　of　dislocations　and　stacking　faults　in　the　structures　caused　by　lattice　distortion.　(C)　2011　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.