Abstract: Ductile iron containing 6.16wt% Al was developed to investigate the effects of aluminum on both its microstructure and hardness. It was found that aluminum not only increases the nodule count and pearlite content but also improves the hardness in both sand mold and metal mold castings. Annealing treatments were conducted to attain a homogenous microstructure and improve high-temperature serviceability. A ferrite/carbide or ferritic matrix was gained depending on the annealing temperature. It is also discovered that annealing has inverse influences on the hardness of the bulk alloy and the ferrite phase. Although it causes a small decrease in the bulk hardness of the specimens, it leads to an increase in the microhardness of the ferrite. Micro-segregation of the alloying elements was also investigated by means of electron probe micro-analysis for the specimens with different annealing durations and the as-cast specimen as well. An optimum annealing time was proposed to result in the least amount of micro-segregation of aluminum and silicon between graphite nodules.