The influence of austempering temperature and time, as main austempering control parameters, on the impact toughness and microstructure of a ductile cast iron alloy having a carbon equivalent of 3.88 percent has been investigated. Two transformation temperatures were chosen at 270°C and 400°C such as to obtain microstructure falling within the zones of lower and upper bainite, respectively. Austempering time intervals ranging from 5 minutes to 5 hours were selected for each austempering temperature. Peak values of U-notch Charpy impact toughness of 7.55 and 17.23 Joule/cm² have been attained at time interval of 60 minutes for specimens austempered at 270°C and 400°C, respectively. The changes in mechanical properties with the transformation temperature and time has been observed to be directly related to the phase transformations in the micro-structure. The study of fracture surfaces of impact toughness samples revealed the great complexity of the crack propagation process in austempered ductile iron due to the nature of its structure.