Austempered ductile cast iron (ADI) has composite microstructures, which are martensite/retained austenite structures. ADI has very large toughness compared to other ductile cast irons. After fracture, the retained austenite near the fracture surface will be transformed to martensite. In the present study, X-ray fractography is applied to fatigue fracture surface of ADI. The fatigue tests were carried out on compact tension (CT) specimens. The volume fraction of retained austenite was quantitatively measured on and beneath fatigue fracture surfaces. The plastic strain on the fracture surface was estimated from measuring the line broadening of X-ray diffraction profiles. The depth of the plastic zone left on fracture surface was evaluated from the distributions of the volume fraction of retained austenite. The results are discussed on the basis of fracture mechanics.