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Machining of industrial parts made from austempered ductile iron (ADI) is both time and money consuming. Therefore, control of dimensional changes during heat treatments of ductile iron as to reduce machining process has received much of interest. In this research austenitization temperature effects on the dimensional changes are studied; the reasons behind these dimensional changes were discussed as well. In this regard, the as cast cylindrical specimens were austenitized at 870, 900 and 920°C followed by austempering at 315°C. Optical and scanning electron microscope(SEM) equipped with EDX and image analyzer were used to explain the reasons of dimensional changes. The results show that increase of austenitization temperature increases the dimensional changes due to increase of volumetric fraction of matrix and porosities around nodular graphites.