Poly(vinylidene fluoride) (PVDF) fibers spun at different take-up speeds (10.6−61.0 m/min) were subjected to stretch−hold deformation at room temperature and in-situ simultaneous synchrotron small- and wide-angle X-ray scattering measurements. Crystal transformation from α to β form and morphological changes in lamellar and fibrillar structures were analyzed in detail. All fibers were found to yield at an early stage of deformation, resulting in alternating necked and unnecked regions along the fiber. From the two-dimensional (2-D) wide-angle X-ray diffraction (WAXD) patterns, mixed α and β (minor fraction) forms were found to coexist in the undrawn fibers. Deformation assisted in the conversion from the α-form into the β-form. In necked regions, more α to β transformation took place than in unnecked regions. The overall crystallinity index and unit cell parameters of the α form did not change significantly prior to necking. From the 2-D small-angle X-ray scattering (SAXS) patterns, two kinds of equatoria...