Poly (ethylene terephthalate) films were treated in supercritical CO2 (s-CO2) fluid, and the modifications in tensile properties were investigated. The yield load peaks became ambiguous and the breaking strains and loads decreased for the treated films. However, the tensile modulus and the yield strain were almost unchanged after the treatments. The yield strain is related to the interplay between the amorphous entanglement density and the stability of crystal blocks. WAXD patterns for the films were hardly modified through the treatment. Thus the orientation for the crystal blocks seems to be unchanged. Moreover, laser Raman microprobe spectroscopic analyses revealed that the orientation of molecular chains is almost retained after the treatments. Thus it is natural to assume that the interplay between the amorphous entanglement density and the stability of crystal blocks will also be kept constant even though the swelling of amorphous regions with s-CO2 occurs during the treatments. From differential scanning calorimetry measurements, it was found that the heat of fusion for the crystallites was a little increased without shifting the peak position after the treatment. Therefore, the increase in crystallinity is not due to the thickening or crystal perfecting for primary lamellae, but due to the secondary crystallization. The increase in crystallinity caused by the secondary lamellae crystallized and combined onto the primary lamellae seems to have reduced the amorphous chains to be deformed easily at tensile tests. Then the breaking strains and loads decreased for the treated films.