We have been investigating in the past the compaction behaviour of powder by particulate modeling in two-dimensions and three-dimensions. This model incorporates plastic deformation of particles at contact besides inter-particle friction, viscous force and gravity. An attempt is made in this study to perform simulation of three-dimensional compaction of powder, where powder is compressed with arbitrary strain rate ratios, and to examine its constitutive behavior, i.e. yield surface and the normality of strain rate vector to the surface. The simulated results show that the shape of the yield surface in principal stress space is a part of an ellipsoid whose axis coincides with hydrostatic stress axis; this qualitatively agree with experimental observations. The normality rule, on the other hand, does not necessarily hold, in particular in a stress state near uni-axial strain compaction.