Highly oriented semicrystalline PET films are exposed to uniaxial statical load both parallel and perpendicular to the preferential orientation direction at various temperatures.The loading of the specimens parallel to the preferential orientation direction leads to an increase in orientation of the chain segments within the amorphous interlayers and to a decrease of density of the amorphous regions which in combination with a rise of the microvoid volume ratio effects a significant increase of the mean square density fluctuation. Thereby chain segements may emerge from the crystalline regions and may pass into the amorphous interlayers.