Abstract Based on an energy approach, the nonlinear elastoplastic behavior of a two-phase, isotropic composite with two kinds of inclusion morphologies are determined as a function of inclusion shape at the low concentration range. Both types of morphology involve the three-dimensional randomly oriented spheroidal inclusions, but one is homogeneously dispersed, resulting in an ordinary two-phase composite, and the other possesses a packeted structure in a form similar to a polycrystalline arrangement. The overall elastoplastic response of these two kinds of composite is found to be strongly dependent upon the inclusion shape and their morphological arrangement. Disc-shaped inclusions generally give a superior reinforcing effect, but when the inclusions become very stiff or totally rigid, needle-shaped inclusions tend to be more effective. In line with the known elastic behavior, the overall elastoplastic response of an ordinary two-phase composite is markedly stiffer than that of the packeted composite at a given inclusion shape and concentration. The theory is finally compared with the finite-element calculations of a particle-reinforced composite and of a packeted one with oblate and prolate inclusions, and also with some experimental data for a silicon-carbide/aluminium system. In both cases, reasonable agreement is found in the comparison.