Crystalline structure and morphology of poly(L-lactide) (PLLA) formed under high-pressure CO2 were studied by comparing the CO2-treated PLLA and the annealed one in terms of the crystallization behavior, crystalline forms, and crystalline superstructures. The crystallization temperature dependence of the diffraction peak position (2 theta approximate to 16 degrees) and crystallinity for the CO2-treated PLLA indicates that the crystal modification changes continuously from the disorder a (alpha") to a. forms not through the alpha' one with increasing temperature. By using light scattering technique, we clarified that the morphological transition from spherulites on a micrometer scale to rodlike crystalline superstructures on a nanometer scale occurs around 15 degrees C under 7-15 MPa CO2 and around 30 degrees C under 3 MPa CO2. By introducing the parameters eta and Delta in the theoretical calculation for Vv light scattering from spherulites, it is indicated that there is a distinct difference in the arrangement of crystalline lamellae within spherulite between the CO2-treated PLLA and the annealed one.