In this study, simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) on as-spun and on spun and subsequently drawn poly(trimethylene terephthalate) (PTT) fibers under cold and hot stretch-hold operations were performed. Crystallinity and orientation analyses were performed on WAXD patterns, while correlation function analysis was used to analyze SAXS patterns to determine the long period. For spun and drawn fibers, WAXD patterns suggested that unit cell dimensions, crystallinity and orientation were dependent on stretching temperature and strain. From orientation analysis, it was reasonable to infer that a rigid amorphous phase existed in the drawn fibers. From SAXS patterns, a defectively stacked lamellar structure was induced at 120 degreesC. Further application of strain up to 100% at 120 degreesC triggered a lamellar to fibrillar transition, as suggested by the formation of a pair of very weak narrow streaks perpendicular to the meridian in the WAXD pattern. These streaks imply a microfibrillar entity. For as-spun fibers, strain could only induce micro-crazes, whose planes are perpendicular to the fiber axis, but could not induce crystallization. At 120 degreesC, WAXD patterns showed that a new transitional phase was formed, which could be transformed to the equilibrium structure with application of strain over 100%.