Phase　transitions　between　the　polytypes　of　boron　nitride　and　the　influence　of　defect　and　impurity　on　preparation　of　c-BN　thin　film　are　studied　from　energy　and　structure　aspects.　A　pathway　from　h-BN　to　c-BN　is　analysed,　namely:　h-BN　-＞　r-BN　-＞　c-BN.　The　transformation　from　h-BN　to　c-BN　is　more　difficult　than　that　from　r-BN　to　c-BN.　The　energy　barrier　is　very　high　in　direct　transformation　from　h-BN　to　c-BN,　but　it　is　very　low　from　r-BN　to　c-BN.　In　fact,　defects　and　impurities　in　c-BN　thin　films　may　favorably　drive　the　transformation　from　h-BN　to　c-BN.　Defects　and　impurities　can　reduce　the　energy　barrier　for　the　transformation　sufficiently　enough　for　it　to　proceed　under　conditions　obtainable　in　common　laboratories.　Based　on　the　theoreticd　model,　we　developed　a　new　method　to　prepare　c-BN　thin　film　(three-step　method).　The　effect　of　time　and　substrate　bias　voltage　on　the　first　step　of　preparation　of　c-BN　thin　film　is　investigated.　The　study　proves　that　5　min　and　-180V　is　very　favorable.　The　c-BN　thin　film　that　content　with　cubic　phase　exceeding　80%　can　be　repeatedly　prepared　using　the　three-step　method.　Results　of　experiments　accord　with　the　theoreticat　model　very　well.