Cold draw processing of poly(ethylene terephthalate) yarns has been discovered that yields a sheath/fibrillar core (s/fc) microstructure in the each fiber of a yarn. When aged, unoriented, noncrystalline spun yarns were cold drawn, high (>5.7 : 1) draw ratios could be achieved and an s/fc microstructure resulted. Cold drawing also generated a high oriented amorphous content. The tensile and shrinkage properties of yarns with the s/fc microstructure and high oriented amorphous content were examined as function of processing (drawing, annealing, and relaxing). The microstructure was examined by scanning electron microscopy, transmission electron microscopy, small-angle X-ray scattering, wide-angle X-ray scattering, and optical microscopy. The fibrillar microstructure survives all processing. Management of the oriented amorphous component under heat and tension contributes to the greater stiffness (modulus) and dimensional stability of processed s/fc yarns. The properties of yarns with an s/fc microstructure are compared with more conventional hot drawn yarns.