Crack　propagation　behavior　on　mullite/fused　silica　interface　and　growing　direction　of　needle-like　mullites　were　investigated　using　diffusion　couples　of　alumina　single　crystal　and　fused　silica．Needle-like　mullites　tended　to　nucleate　more　easily　on　（5507）　of　the　alumina　single　crystal，which　seemed　to　be　attributed　to　the　similarity　of　the　atomic　configuration　between （001） of　mullite　and （5507） of　alumina．The　crack　passing　through　the　needle-like　mullite　was　found　to　be　propagated　along （001） of　mullite．This　musｔ　be　originated　in　the　strict　bonding　of　mullite/fused　silica　interface　and　the　generation　of residual　tensile　stress　due　to　the　mismatch　of　thermal　expansion　between　mullite　and　silica．

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　The　quantitative　analysis　about　the　thermal　shock　resistance　of　Al2O3－C　brick　（submerged　nozzle　brick）　was　attempted．Main　data　are　effective　fracture　energy，modulus　of　static－elasticity，tensile　strength　and　linear　thermal　expansion　etc．at　the　temperature　range　between　R．T．and　1500℃．Effective　fracture　energy　was　measured　with　a　chevron　notched　sample　by　work　of　fracture．Modulus　of　static－elasticity　was　calculated　from　a　deflection　curve　in　modulus　of　rupture　test．In　each　test，the　displacement　was　measured　by　the　non－contact　method．Tensile　strength　was　measured　by　the　disk　test．And　then，the　nozzle　sample　was　submerged　in　the　melting　metal　of　800℃．In　that　test，the　occurence　time　of　the　accoustic　emission　wave　was　measured．From　the　result，the　thermal　stress　was　calculated　by　the　finite　element　method、 　We　found　that　the　check　of　the　test　method　and　the　test　condition　was　more　important　in　quantitative　analysis　than　these　in　qualitative　analysis．We　found　that　if　Rst＞30～40（m1/2・℃）　（in　case　of　about　30mm　thickness　in　consideration　of　the　size　effect）　K₁c　＞2～1（MN・m-3/2），and　tesile　strength＞22（㎏f／c㎡）（at　1000℃）　in　our　samples，the　thermal　shock　resistance　was　large．

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