This work is devoted to the prediction of the constitutive steady-state creep behavior of matrix inclusion composites. Both phases are characterized by power-law constitutive equations. The three phase model is extended to viscoplastic equations. If both phases have the same strain rate sensitivity, the effective behavior of the composite is characterized by an effective prefactor. If not, an effective strain rate sensitivity is defined, which is a function of the applied strain rate and of the volume fraction of the phases. All the results are compared with the classical self-consistent ones. A limit case which may be related to the grain boundary sliding accommodated by intragranular power-law creep is also studied.