Austempered ductile cast iron (ADI) which has been recently developed shows good mechanical properties. These properties are related to the microstructure which is greatly affected by processing variables such as austempering time and temperature. In this study, the relationships between mechanical properties from impact test, and hardness test and the results from ultrasonic velocity measurement and electrical resistivity measurement are studied on the ADI samples which are heattreated at different austempering temperature and time. From the results, we conclude the followings. The ultrasonic velocity measurement could be used for the study of austempering reaction mechanism. The electrical resistivity measurement could be used as quality assurance technique for the ADI.