This work presents a micromechanical model to investigate mechanical properties of nanotwinned dual-phase copper, consisting of the coarse grained phase and the nanotwinned phase. Both strengthening mechanisms of nanotwinning and the contributions of nanovoids/microcracks have been taken into account in simulations. With the aid of modified mean-field approach, the stress–strain relationship is derived by combining the constitutive relations of the coarse grained phase and the nanotwinned phase. Numerical results show that the proposed model enables us to describe the mechanical properties of the nanotwinned composite copper, including both yield strength and ductility. The calculations based on the proposed model agree well with the results from finite element method (FEM). The predicted yield strength and ductility are sensitive to the twin spacing, grain size, as well as the volume fractions of phases in this composite copper. These results will benefit the optimization of both strength and ductility by controlling constituent fractions and the size of the microstructures in metallic materials.