In this study, we investigated the effect of Sb/RE on the microstructure and mechanical properties of as-cast heavy sectioned, over 250mm thickness, ferritic ductile cast iron. Exothermic and thermal insulation material were equipped on the wall of sand cast mold having the dimensions of $250{ imes}250{ imes}250$ mm. The nominal composition of the molten metal was controlled to be on the eutectic composition and Sb was added about 0, 0.005 and 0.02% respectively. In the center of as-cast ingot without Sb addition, the solidification of chunky graphite was induced by the eutectic reaction that took long time, which caused the decrease of elongation and impact energy. In case that the value of Sb/RE is 0.8, the solidification of chunky graphite could be suppressed and the improvement of nodularity was observed. On the other hand, the excessive addition of Sb suppressed the solidification of chunky graphite but gave rise to the solidification of flake graphite and the increase of pearlite contents. This results in poor elongation and impact energy which is lower than those in the case of no Sb addition.