The atomistic-continuum model was validated for a heterogeneous material, and elastic constant of semicrystalline poly(trimethylene terephthalate) (PTT) was calculated by the model. Perfect agreement between simulated strain energy and its theoretical value was observed for a two-phase series model for composites, as the energy minimization proceeded, indicating that the model properly describes heterogeneous materials. The system for semicrystalline PTT consisted of a matrix (amorphous phase), described as a continuum, and a spherical inclusion (crystalline phase), represented in atomistic detail. The degree of crystallinity of semicrystalline PTT could be controlled by varying the volume fraction of an inclusion. The Young's moduli in all directions of semicrystalline PTT increase with increasing the degree of crystallinity, as expected.