The fracture properties obtained in austempered ductile iron castings are dependent on metallurgical process variables, including chemical composition and heat treatment conditions. In the present work alloyed ductile irons of 0.27%Mo-1.08%Ni and of 0.24%Mo-0.57%Cu were studied for their response to austempering. The alloys were austenitized for 1.5 hrs at 850-1000℃ and austempered for 2 hrs in salt at 385℃. The optimum mechanical properties and fracture toughness were obtained when the alloys were austenitized at 900-950℃.