In this study, mechanical and magnetic properties of high silicon ductile irons (GJS 500-14-and GJS 600-10) with fully ferritic initial microstructures were investigated after being austempered at 270-390 degrees C for 60 min. Results showed that hardness and strength as well as ductility are mainly controlled by morphologies and volume fractions of the phases in the austempered microstructures. More silicon in the chemical composition results in a significant decrement in ductility and toughness. For this reason, when high toughness is concerned, GJS 600-10 grade ductile iron containing more silicon should not be a preferred grade for austempering. In the view point of magnetic properties, saturation magnetization strongly depends on the austempering temperature, whereas remenance and coercivity are somewhat related to mean ferritic cell size and the solute atoms in the chemical composition.