The effect of zirconium (Zr) inoculant on ductile iron microstructure was investigated. It was found that Zr-containing post inoculant generated high graphite nodule counts and refined the austenite dendritic structure as well. EBSD, a microstructural-crystallographic characterization technique, was utilized to study the mechanism of the high nodule counts by using water-quenched samples. It was found that the effect of Zr on nodule count was not due to the nucleation of graphite but the refinement of austenite dendritic structure. In this study, direct and precise measurement of the austenite grain size was feasible only when higher quenching temperature was used (1150C). This is because the solubility of carbon in austenite increases with higher quenching temperature, bringing the Ms point down below room temperature.