Tensile deformation behavior of a random propylene ethylene copolymer with 12 mol % ethylene counits at room temperature and 63 degrees C was investigated using the in situ small and wide-angle X-ray scattering techniques. Under both conditions, the deformation mechanism changed from slip of crystalline lamellar blocks to stretching induced melting and recrystallization process at a critical strain of about 0.9. This critical strain in tensile deformation of semicrystalline polymers normally marks the starting of plateau value of elastic strain. Further stretching leads to increase of plastic deformation only due to the fibrillation. However, a peculiar elasticity reinforcement was observed at strain larger than 1.3 when the sample was stretched at 63 degrees C. Wide angle X-ray scattering results indicate that at this strain of 1.3 fibrillation of the originally unoriented crystals finished so that further stretching leads to a deformation of a rather stable entangled network embedded by fibrils that possesses high elasticity.