To obtain highly transparent isotactic polypropylene sheet by an industrial process, various factors contributing to transparency were analyzed. Two kinds of sheets were prepared for analysis. The one was extruded by a direct single belt process as the quick quenching system. And the other was extruded by a conventional air knife process as the slow cooling system. The higher order structure of each sample sheet was investigated by phase-contrast microscopy, polarizing microscopy, light scattering, wide-angle X-ray diffraction (WAXD), small angle X-ray scattering (SAXS), and transmission electron microscope (TEM). In the case of the quick quenching sheet, many spherical substances were generated near sheet surfaces. Comparing with static cooling experiments, it was thought that the main cause was the orientation crystallization by the residual stress. The spherical substances in a phase-contrast micrograph were extremely coincident with the maltese cross pattern in a polarizing micrograph. Other analysis results also showed that these spherical substances were spherulites. And after the heat treatment, the matrix was transformed from smectic structure to alpha-monoclinic crystal phase. The density and refractive index differences between spherulite and matrix were decreased, and the transparency was drastically improved. In addition, some guideline for the improvement of transparency was indicated. POLYM. ENG. SCI., 50:2356-2365, 2010. (C) 2010 Society of Plastics Engineers