The effect of CO2 on the isothermal crystallization kinetics of poly(L-lactide), PLLA, was investigated using a high-pressure differential scanning calorimeter (DSC), which can perform calorimetric measurements while keeping the sample polymer in contact with pressurized CO2. It was found that the crystallization rate followed the Avrami equation. However, the crystallization kinetic constant was changed depending upon the crystallization temperature and concentration of CO2 dissolved in the PLLA. The crystallization rate was accelerated by CO2 at the temperature in the crystal-growth rate controlled region (self-diffusion controlled region), and depressed in the nucleation-controlled region. CO2 has also decreased the glass transition temperature, T-g, and the melting, temperature, T-m. As a result, the CO2-induced change in the crystallization rate can be predicted from the magnitudes of depression of both T-g and the equilibrium melting temperature. The crystalline structure and crystallinity of polymers crystallized in contact with pressurized CO2 were also investigated using a wide angle X-ray diffractometer (WAXD). The resulting crystallinity of the sample was increased with the pressure level of CO2, although the presence of CO2 did not change the crystalline structure.