Six groups of castings which solidified at different cooling rates with or without addition of 0.03%Sb were fabricated. The effects of the melt holding time, cooling rate, and Sb on the microstructure of ductile iron were studied. Results show that as for the other three blocks with addition of 0.03%Sb, reducing the length of eutectic plateau could improve the graphite morphology, and sufficient residual contents of spheroidizing elements REE and Magnesium could also lead to small and diffuse globular graphite particles no matter what the cooling rate is. As for the other three blocks with addition of 0.03%Sb, no matter what the cooling rate is, the nodularity index values are all about 90%. The main difference among the three blocks is just the size of nodular graphite. And even if two of the blocks solidified at different cooling rates, they own nearly identical NI value and globular graphite average diameter value, which is because they both possess the tolerated rare-earth elements/antimony ratio for spheroidal graphite formation.