Changes in the lamellar and crystalline structures were followed as a function of Applied stress to understand the influence of the interactions between the crystalline and amorphous domains on the fiber properties. We observed a reversible transformation from a structure giving a four-point small-angle pattern to a structure giving. a two-point pattern; these structures corresponded to the lamellae with oblique and normal lamellar surfaces respectively. The characteristics of these two structures such as the stack diameter, stack height, and tilt angle were different and were determined by the, processing conditions and did not change when the fiber was elastically deformed. The structure giving a two-point pattern was probably the load-carrying lamellar entity in these fibers. The diameter of the lamellar stacks,. tilt angle of the lamellae, And the strain, in the, lamellar spacing appeared to have the most influence on properties such as tenacity and dimensional stability. The long-spacing strain, Which is about the same as the fiber strain, determined the modulus at low elongation as: well As ultimate elongation. These indicate that the lamellar stacks have At least as much influence as the interfibrillar chains on fiber properties. Structural features that determine the performance in semicrystalline polymers were investigated by, analyzing four generations of polyethylene terephthalate fibers. Some of the fiber properties correlate with changes in the crystalline domains such as the crystalline orientation, sized and unit cell dimensions. Fibers in which the crystalline strain was large because of their strong linkages to the amorphous chains, and better load transfer, had the highest modulus and lowest ultimate elongation. (C) 2003 Wiley Periodicals, Inc.