Statistics for A ductile iron was austempered at 302 and 385°C for various times to get lower and upper ausferrite microstructures respectively. The microstructures were characterised by optical microscopy and X-ray diffraction. Plane strain fracture toughness was determined under all heat treatment conditions. While the austempered ductile iron with lower ausferrite microstructure showed higher fracture toughness, the one with upper ausferrite microstructure exhibited higher tensile toughness and strain hardening coefficient. A model was developed relating fracture toughness to the yield strength (?<inf>y</inf>) volume fraction of retained austenite (X<inf>?</inf>) and the carbon content of the retained austenite (C<inf>?</inf>). Experimental results showed excellent agreement with the prediction of the model that K2<inf>1C</inf> is proportional to ?<inf>y</inf>(X<inf>?</inf>C<inf>?</inf>)1/2.