On-line studies of structure and morphology changes in Kevlar 49 fibers during stretching were carried out using synchrotron simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. A unique two-dimensional (2D) image analysis method was used to extract quantitative information of crystal, amorphous and mesomorphic fractions from 2D WAXD patterns. Results showed that about 20% of the fraction (mass) in the Kevlar 49 fiber was mesophase, 50% was crystalline and 30% was amorphous. There were transitions between crystal, amorphous and mesomorphic fractions during deformation. The crystal orientation was obtained in terms of the Herman's orientation function f(2) from the azimuthal scan of the (200) crystal reflection. The crystal orientation was found to be quite high in the Kevlar 49 fiber and increased with the stretch ratio. The fibril length and misorientation were also obtained from 2D SAXS patterns by using the Ruland method. Results showed that the fibril length decreased with the stretch ratio until 2.0% and then increased. The misorientation decreased with increasing stretch ratio.