Deformation behavior of polytetrafluoroethylene (PTFE) films was investigated by thermomechanical analysis (TMA) under various tensile stresses (sigma) up to 1.15 MPa in the temperature range from room temperature to 360 degrees C. In the heating process above sigma approximate to 0.25 MPa, a contraction of the PTFE film occurs in the melting temperature region. In the cooling process above sigma approximate to 0.05 MPa, an elongation occurs in the crystallization temperature region, and above sigma approximate to 0.5 MPa, it reaches 20-30% of the original length of the film. The PTFE films in the melt state above sigma approximate to 0.5 MPa contract with increasing temperature up to 360 degrees C and elongate with decreasing temperature. For the films that underwent deformation in the TMA, the crystalline orientation and the surface morphology were investigated by wide-angle X-ray diffraction and scanning electron microscopy, respectively. The degree of crystalline orientation in the deformed films increases with increasing sigma and approaches a plateau at sigma approximate to 0.4 MPa. On the surface of the deformed films, alignment of the bands and deformation of granules, which are formed by heat treatment above the melting point, are observed.