The　property　of　labyrinth　seal　relies　on　the　throttling　process　in　the　clearances　and　the　kinetic　dissipation　process　in　the　cavities　just　behind　the　clearance,　these　two　procedures　play　decisive　roles　in　essential　to　the　sealing　property　of　the　labyrinth.　Study　on　the　internal　flow　pattern　of　labyrinth　seal　is　helpful　for　us　to　make　use　of　some　characters　of　turbulence　in　improving　the　sealing　property.　In　the　present　paper,　the　flow　field　in　a　crenellated　radial　labyrinth　seal　is　numerically　studied,　the　flow　pattern　is　obtained,　and　the　relationships　between　the　turbulent　flow　in　the　seal　and　the　sealing　properties　are　revealed.　The　numerical　results　show　that　there　is　an　optimal　teeth　insert　length　for　the　sealing　property.　Under　the　condition　of　the　optimal　teeth　insert　length,　although　the　throttling　clearance　is　large,　the　inertial　deflection　and　contraction　of　the　jet　flow　through　the　clearance　decreases　the　effective　flow　area　of　the　clearance,　thus　the　discharge　medium　can　obtain　higher　velocity　in　the　clearance　and　proper　dissipating　vortex　can　be　formed　in　the　cavity,　so　the　seal　reaches　the　best　sealing　property.　We　can　conclude　that　by　making　use　of　the　resistance　enhancement　by　turbulence,　better　sealing　property　can　be　achieved　under　the　condition　of　large　clearance,　which　is　of　great　importance　to　the　design,　manufacture　and　operation　of　labyrinth　seal,　and　lays　the　theoretical　foundations　for　new　labyrinth　seal　techniques.