Solidification　structure　of　two　kinds　of　B　alloyed　high　speed　steel　for　rolling　mill　rolls,　which　carbon　contents　are　0.50　%　and　1.00　%,　respectively,　and　the　changes　in　the　microstructure　and　mechanical　properties　after　quenching　from　1050°C　and　tempering　at　200-600°C,　respectively,　are　studied.　The　results　of　metallographic,　scanning　electron　microscopy　analysis,　X-ray　diffraction　analysis,　hardness　and　impact　toughness　measurements　of　as-cast　and　heat-treated　B　alloyed　high　speed　steel　are　discussed.　As-cast　matrix　of　B　alloyed　high　speed　steel　consists　of　martensite　and　troostite.　There　are　19-26　vol.%　M23(B,　C)6,　M3(B0.7C　0.3)　and　M2(B,　C)　type　carboborides　in　the　matrix.　Microhardness　values　of　M23(B,　C)6,　M3(B　0.7C0.3)　and　M2(B,　C)　are　1940-2030　HV,　1380-1460　HV　and　1460-1530　HV,　respectively.　Macrohardness　of　as-cast　B　alloyed　high　speed　steel　reaches　58-60　HRC.　After　quenching　from　1050°C,　the　eutectic　M23(B,　C)6　and　M3(B　0.7C0.3)　carboborides　dissolve　into　the　matrix,　and　many　granular　M23(B,　C)6　precipitate　from　the　matrix,　and　the　whole　matrix　transforms　into　martensite.　Microhardness　of　matrix　and　macrohardness　of　quenched　B　alloyed　high　speed　steel　have　a　slight　increase　comparing　with　as-cast　sample.　Hardness　of　the　B　alloyed　high　speed　steel　remains　constant　while　tempering　temperature　is　below　550°C,　and　then,　with　increasing　temperature,　decreases　considerably.　Impact　toughness　of　the　B　alloyed　high　speed　steel　increases　slightly　until　tempering　temperature　reaches　525°C,　and　then　increases　considerably.　B　alloyed　high　speed　steel　has　higher　hardness　and　has　an　excellent　wear　resistance　after　tempering　under　550°C　that　can　be　attributed　to　the　effect　of　boron.