The melting behavior and crystalline forms of isotactic polypropylene (iPP) samples crystallized under different conditions of pressure and temperature were investigated using differential scanning calorimeter (DSC) and wide-angle X-ray diffraction (WAXD), respectively. When treated with dynamic supercritical CO2(Sc-CO2), iPP samples undergo the formation of beta-crystal that does not occur on the treatment with atmospheric pressure and static supercritical CO2(Sc-CO2) pressure. In addition, the relative content of beta-crystal has deep dependence on melt state and depressurization rate. Depressurization plays very critical role in the formation of beta-crystal by means of imposing three-dimensional tensile field during cell growth. The tensile field induced alpha-row nuclei where the formation of beta-crystal occurred. This finding will provide one new method to induce beta-crystal in iPP parts. (C) 2016 Society of Plastics Engineers