AbstractSmall‐ and wide‐angle X‐ray scattering (SAXS and WAXS), shrinkage, and density experiments were performed on poly(butylene terephthalate) fibers which had been isothermally crystallized at different temperatures, and at constant tension, for times ranging from 100 to 1050 ms.A consistent correlation among WAXS, SAXS, and the kinetic results is demonstrated. Shrinkage results show that the crystallization process prevails over the chain‐re‐coiling process. Density measurements show that the rate of crystallization increases with temperature. Pinhole X‐ray photos show that the orientation of the chains within the crystals remains constant with time and temperature. WAXS diffractometer scans show the development of wide‐angle Bragg peaks.SAXS shows the development of small‐angle Bragg peaks, as the annealing time is increased. The two‐lobe arced pattern is the characteristic pattern. The value for long spacing ranged from 100 to 120 Å, increasing with temperature.