Microstructure　and　properties　of　isothermally　quenched　high　boron　white　cast　iron　were　investigated　in　this　paper.　The　results　show　that　the　microstructure　of　high　boron　white　cast　iron　is　mainly　composed　of　many　continuous　and　netlike　eutectic　borides,　pearlite　and　ferrite　under　as-cast　condition.　The　microhardness　of　Fe2B　ranges　in　1200-1600HV　whose　value　seems　to　approximate　that　of　(Fe,Cr)(7)C-3　type　carbide　(HV1200　similar　to　1800)　in　high　chromium　white　cast　iron.　After　isothermal　quenching,　the　matrix　transforms　into　lower　bainite　in　which　carbide　precipitations　are　arranged　in　parallel　rows　at　an　angle　of　60　deg　to　the　long　axis　of　the　plates,　but　the　morphology　of　boride　remains　nearly　unchanged　compared　with　its　as-cast　condition.　Moreover,　precipitation　particles　with　the　size　of　about　1　similar　to　4　mu　m　can　be　found　in　the　matrix　of　isothermally　quenched　high　boron　white　cast　iron.　Impact　fracture　morphology　of　isothermally　quenched　high　boron　white　cast　iron　indicates　that　fracture　propagated　more　easily　through　boride/matrix　interface　than　through　matrix.