In light of the feeding principle for ductile iron (DI) castings proposed by Karsay, the three-dimensional (3-D) numerical simulation of the volume variation of a DI casting during the solidification process is carried out. Using the results of simulation, a plot of the apparent density of the casting, versus the time after pouring, is made. According to the plot, the degree of shrinkage defect can be evaluated from the time when the riser neck is blocked. In view of the degree and position of shrinkage defect in the castings, the calculated results coincide with those of the experiments, basically.