Off-line small- and wide-angle X-ray scattering and optical birefringence measurements on partially oriented yarns (POY) of poly(trimethylene terephthalate) prepared at different take-up speeds suggest the formation of a rigid amorphous phase for intermediate take-up speed POY. Simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques were utilized to perform an in situ study of structure development of these POY during continuous hot-drawing. From SAXS patterns, it is suggested that a lamellar stacking arises at temperature above 100 degreesC during drawing. A model of lateral growth of crystallites with extended chains as nucleation sites is suggested. The microfibril length and long period of the lamellar stacking depend upon the preparation take-up speeds of POY and draw ratio. From WAXD patterns, it is observed that crystallinity developed above 100 degreesC with draw ratio 1:1 is temperature controlled. Both orientation and unit cell dimensions were extracted from WAXD patterns. At 100 degreesC, while orientation increases with draw ratio, the unit cell dimensions, especially the dimension of the c-axis, and the crystallinity depend on both draw ratio and take-up speeds. This dependence can be explained by a transformation process coupled by defect creation/transport and crystallization.