Elasto-plastic constitutive relation is derived from Doraivelu's yield function. The relation is implemented as user subroutines of MSC. Marc, a commercial FEA code. The compaction process of a brick-shaped specimen is numerically simulated. Density distribution obtained shows that largest gradient developed in corners. And although the upper half is more compact than the lower half, the density at the center of the upper surface is smaller than that at the center of the lower surface. Load vs. displacement curves obtained from simulation and experiments are compared. Both curves show that most of the densification occurred in the initial stage of the compaction process. The comparison indicates that the constitutive relation used here is stiffer than the actual powder material in the initial stage, while turns to be close to it at last.