Synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) were used to characterize the structure evolution of propylene-1-butylene (P-B) random copolymer subjected to uniaxial tensile deformation at 100 degrees C. Polymorphism and preferred orientation of the crystal phases were examined quantitatively by 2D WAXD. The results indicated that three ensembles of crystalline modifications with distinctive orientation modes coexisted during stretching. The orthorhombic gamma-form adopted a tilted cross-beta configuration, in which the c-axis had a tilt angle with respect to the fiber axis. The monoclinic alpha-form in the mother lamellae had a c-axis orientation with polymer chains parallel to the fiber axis. In the alpha-phase daughter lamellae, the unit cell assumed an a-axis orientation, where the c-axis had an 80 degrees angle with respect to the fiber axis. Stretching transformed the gamma-phase into the energetically more stable alpha-phase. In the late stage, the system was dominated by the alpha-phase with parallel chain packing. Complemented by qualitative SAXS analysis, simultaneous inter- and intralamellar chain slips were observed during the early stage of stretching. After yielding, a fibrillation process followed. The formation of fibril bundles together with a cross-linked network after yielding might account for the stress-hardening behavior in the late stage of stretching.