An　optimized　microstructure　of　cast　iron　has　been　developed.　In　situ　studies　with　the　use　of　scanning　electron　microscopy　have　been　made　of　the　microprocesses　of　deformation　and　fracture　in　tension.　The　surface　microstructure　and　the　corresponding　fracture　surface　and　matching　fracture　were　observed.　The　optimized　nodular　graphite-martensite　interface,　in　combination　with　the　high　yield　strength　martensite　shell,　has　been　found　to　delay　the　initiation　and　propagation　of　interfacial　cracks.　Subsequent　propagation　and　the　formation　of　cast　iron　cracks　is　hindered　by　the　ferrite-plus-martensite　matrix.　During　crack　propagation,　shear　deformation,　the　fracture　of　ligaments　between　the　crack　tip　and　the　microcracks　ahead,　crack　deflection,　and　bridging　take　place.　With　the　use　of　these　observations,　it　is　possible　to　explain　why　cast　iron　with　this　optimized　microstructure　has　excellent　strength　and　toughness.