A　measurement　of　fatigue　damage　before　crack　initiation　in　austempered　ductile　iron　under　three-point-bending　cyclic　loading　was　carried　out　by　using　system　analysis　method.　The　parameter　of　fatigue　damage　is　deduced.　The　test　results　show　that　separation　at　interface　between　graphite　and　matrix　occurs　firstly　under　cyclic　loading,　however,　the　separated　interface　do　not　always　grow　into　fatigue　cracks.　In　more　cases,　the　separated　interface　can　promote　the　formation　of　slip　band　and　the　initiation　of　fatigue　cracks　in　the　surrounding　matrix.　The　development　of　the　slip　band　into　fatigue　cracks　is　observed.　The　test　results　indicate　the　existence　of　fatigue　strength　of　interface　between　graphite　and　matrix　in　austempered　ductile　iron.