Transient structure and crystallization behaviors in cold-drawn PET fibers were studied by synchrotron wide-angle X-ray diffraction, IR spectroscopy, and thermomechanical analysis. As-spun PET fibers were mechanically cold-drawn up to the breaking point, and those stretched ones beyond the strain hardening point in the stress-strain curve exhibited the transient layer structure. Evidence suggesting the tilted PET chains within the transient layer structure, and therefore the smectic C mesophase, was discussed. The tilt angle of the chains within the layer was 6degrees against the fiber axis. The tilt angle, however, decreased rapidly with the crystallization. Results of X-ray diffraction, IR absorbance, and thermomechanical analysis suggested two-stage crystallization in oriented PET chains. Bundles of highly oriented chains including the mesophase were responsible for the first stage crystallization (80-100 degreesC), but those of less ordered or nonoriented chains were for the second stage crystallization beyond 140 degreesC. The structural transformation from the transient mesophase to the triclinic crystal structure was interpreted by the chain sliding mechanism, leading to the tilted (001) planes against the fiber axis.