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　degrees　C　and　tempering　at　200-600　degrees　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.%　M-23　(B,　C)(6),　M-23　(B0.7C0.3)　and　M-2(B,　C)　type　carboborides　in　the　matrix.　Microhardness　values　of　M-23　(B,　C)(6),　M-23(B0.7C0.3)　and　M-2　(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　degrees　C,　the　eutectic　M-23　(B,　C)(6)　and　M-23　(B0.7C0.3)　carboborides　dissolve　into　the　matrix,　and　many　granular　M-23　(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　degrees　C,　and　then,　with　increasing　temperature,　decreases　considerably.　Impact　toughness　of　the　B　alloyed　high　speed　steel　increases　slightly　until　tempering　temperature　reaches　525　degrees　C,　and　then　increases　considerably.　B　alloyed　high　speed　steel　has　higher　hardness　and　has　an　excellent　wear　resistance　after　tempering　under　550　degrees　C　that　can　be　attributed　to　the　effect　of　boron.