Tensile-deformation behavior and structure change of isotactic polypropylene (iPP) under pressurized CO2 were investigated by using in situ tensile-deformation instruments and small-angle X-ray scattering (SAXS) measurements. Tensile strength including yield stress, plateau stress, and tensile modulus decreased with increase of CO2 pressure, suggesting that expansion of amorphous regions by absorbing CO2 allows crystalline lamellae to deform easily with less entanglement. Under CO2, the specimens became opaque beyond yield point and strong equatorial streaks appeared in two-dimensional SAXS patterns. By using Ruland's streak method, they are attributed to formation of nanometer-sized voids larger than the periodic distance of crystalline lamellae. An azimuthal profile of the lamellar peak was sharper under pressurized CO2 than that under ambient pressure. This meant that the easy rotation of the crystalline lamellae by CO2 results in the preferred orientation of the crystalline lamellae, which caused formation of the nanometer-sized voids at the interspace in the polymer matrix between lamellar stacks. Owing to the formation of the nanometer-sized voids, the elongation was enhanced in the second stage stretching of the stretched iPP at ambient pressure after depressurization. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013