The　microprocesses　of　crack　initiation　and　propagation　in　an　austempered　ductile　iron　have　been　investigated　by　using　an　scanning　electron　microscope　with　a　microtensile　holder,　It　was　revealed　that　microcracks　always　initiated　at,　and　propagated　along,　the　graphite-matrix　interface.　Microcracks　in　the　matrix　initiated　after　the　interface　cracking.　The　graphite　nodules　in　this　cast　iron　could　not　be　regarded　as　voids　with　no　strength　because　internal　fracture　of　the　graphite　was　observed.　When　the　cracks　propagated　in　the　matrix,　they　often　grew　along　the　bainitic　ferrite-austenite　interface,　Crack　deflection,　shear　ligament　formation,　and　crack　branching　occurred　during　the　connection　of　main　cracks　and　microcracks,　which　contributed　to　the　strength　and　toughness　of　the　ADI.　(C)　2001　Elsevier　Science　B.V.　All　rights　reserved.