A new numerical method to simulate the centrifugal process of fabricating functionally graded materials (FGMs) from solid-particles/viscous-matrix mixtures is proposed, and the simulation method was successfully applied to a practical fabrication process of FGM from an alumina-fillers/epoxy-resin mixture. Gradient profiles of dielectric constant of the resultant FGM were estimated by using the proposed method and compared with the experimental ones, resulting in reasonable agreement between them. Based on the numerical results, gradient profiles of the number density and size of the dispersed fillers were confirmed, and contribution of the filler size toward the gradient in the packing fraction was demonstrated. It is concluded that the gradient in the filler distribution can be intentionally regulated by changing not only the centrifugal conditions, but also the size distribution of the fillers.