Microstructure and mechanical properties of ductile cast iron were investigated in terms of diameter change of samples that gives rise to modify the microstructure due to different cooling rate in the continuous casting process. The chemical composition used in this study was GCD 400 grade. From the microstructural observation, we have found a large number of graphite with small size in diameter which is comparable to the microstructure of the sample produced by conventional sand casting. The major reason of this would he due to high cooling rate. In the sample with 26 ㎜ in diameter, the microstructure was composed of pearlite, iron carbide, and graphite. In the samples with 60 and 100 ㎜ in diameter, however, we have observed a dissimilar microstructure that consisting of ferrite and graphite. Concerning the mechanical property, the sample with 26 ㎜ in diameter showed higher hardness and strength compared to those samples with 60 and 100 ㎜ in diameter. The result obtained for ductility appeared a reversal. Much more works such as inoculation, process design and chemical composition would be required in order to have a sound product even in a small diameter of samples.