We have prepared poly(ethylene-2,6-naphthalate) (PEN) films having higher-order structure with fine crystallites using a supercritical carbon dioxide (SCCO2) treatment technique at a relatively low-temperature range, from 110 to 170 degrees C. After the SCCO2 treatment, the glass transition temperature T-g decreased by more than 50 degrees C, and the PEN films were crystallized. Long-term change in the FT-IR absorbance intensity at 2335 cm(-1) (C=O) was in good agreement with the change in T-g. The large decrease in T-g was attributed to the sorption of the CO2 molecules into the PEN film. Moreover, the increase in the treatment pressure increased the amount of absorbed CO2 and reduced T-g, which can promote crystallization. The higher-order structure, long period, lamella, and interface thickness of the crystallized PEN films were calculated from the one-dimensional correlation function by SAXS. These parameters were all linearly related to the treatment temperature. The crystallite size obtained from the Scherrer equation decreased with decreasing treatment temperature. It is suggested that the higher-order structure of PEN is decided only by the treatment temperature and that the SCCO2 treatment promotes the creation of the nuclei in the amorphous state at low temperatures, following the formation of fine crystallites.