The application of this new design approach called fracture mechanics allow one to determine the maximum allowable stress from the knowledge of the largest expected flow size and the plane strain fracture toughness of a material. In this study we examined the relation between retained austenite, mechanical property and fracture toughness accompanied by austempering heat treatment. Fracture toughness values and retained austenite volume were higher with the ADI (austempered ductile iron) which were austempered at 380℃ than austempered at 320℃. Additionally, fracture toughness values were increased for 1∼2hour austempering time but it was slowly decreased for 5hour ADI maintaining the predominant fracture toughness(Kic:83㎫√m) is obtained following condition, namely, austempering temperature and time (380℃ and 1hour).