The elastic responses of crystalline and amorphous parts in semi-crystalline nylon 6 have been determined by computer simulation using the finite element method. Semi-crystalline nylon 6 has been modelled as a composite consisting of alternating layers of lamellar crystals and amorphous regions. Full morphological details identified previously by Lin and Argon in highly textured nylon 6 bulk samples have been incorporated in the model. An optimization scheme has been employed to search systematically for the individual components’ elastic constants which give rise to a composite elastic behaviour as that measured by Lin and Argon. A two-dimensional plane strain finite element analysis has been performed to evaluate the composite elastic behaviour for a given set of constituents’ elastic constants. The resulting elastic constants of semi-crystalline nylon 6 for the optimized values of crystalline and amorphous elastic properties were within 6% average error with the experimental data. The computations also revealed that a high stress concentration exists in the crystalline region. Therefore, experimental measurements of plastic resistance may represent a significant underestimate of the intrinsic critical resolved shear strength of polymer crystals.