We investigated the crystallization growth of isotactic polypropylene under carbon dioxide (CO2) at various CO2 pressures and temperatures by in situ observation with a digital high-fidelity microscope and a specially designed high-pressure visualized cell. The fibrils within the spherulite were distorted and branched by crystallization under CO2 at pressures higher than 2 MPa, and this suggested the exclusion of CO2 from the growth front of the fibrils. The spherulite growth rate (G) at 140degreesC increased with the CO2 pressure, attained a maximum value around 0.3 MPa, and then decreased. Above 6 MPa, it became slower than that under air at the ambient pressure. An analysis of the crystallization kinetics by the Hoffman-Lauritzen theory revealed that the pressure dependence of G could be ascribed to the change in the transportation rate of crystallizable molecules (beta(g)) with pressure; that is, beta(g) increased and then decreased with pressure. The increase in beta(g) at a low pressure was caused by the plasticizing effect of CO2 whereas the decrease in beta(g) at a high pressure was due to the exclusion of CO2 from the crystal growth front. (C) 2004 Wiley Periodicals, Inc.