The objective of this study is to develop a unified, three-dimensional micromodel which can describe the nonlinear elastoplastic constitutive behavior of MMC’s with continuous fibers, particles, or aligned short fibers as the reinforcement. In the micromodel, both the reinforcement and the matrix may have elastic and/or elastoplastic deformation(s), respectively. However, as in most cases, the development is shown for elastic reinforcement in an elastoplastic matrix. The micromodel uses a repeating unit-cell model with eight subcells. For a particulate or an aligned short fiber composite, one center subcell represents the particle or the short fiber while the rest of the subcells represent the matrix. On the other hand, two aligned subcells indicate the fiber for a fibrous composite. The micromodel yields the overall effective constitutive equation for an MMC with elastoplastic deformation in the matrix material. The effective stress-strain plots of various MMC’s are predicted using the micromodel for a wide variation of the reinforcement volume fraction. These results are compared to those obtained from the finite element analysis, and the two results agree very well.