The deformation behavior of homogeneous ethylene-1-octene copolymers was investigated as a function of the crystallinity and the crystal size and perfection, respectively, by wide- and small-angle X-ray scattering using synchrotron radiation. The crystallinity and the crystal size and perfection, respectively, are controlled by the copolymer composition and the condition of melt crystallization. The deformation includes rotation of crystals, followed by plastic deformation and complete melting of the initial crystal population, and final formation of microfibrils. The process of rotation, plastic deformation, and melting of crystals of the initial structure is completed at lower strain if the size and perfection of the crystals, respectively, decrease, that is, if crystals thermally melt at lower temperature. The kinetics of the fibrillation of the initial structure seems independent of the crystal symmetry, that is, rotation and melting of pseudohexagonal and orthorhombic polyethylene crystals (as evident in low-crystalline specimens) are similar. The structure of the microfibrils, before and after stress release, is almost independent of the condition of prior melt crystallization, which supports the notion of complete melting of the initial crystal population.