An in situ study of strain-induced crystallization in an amorphous poly(ethylene terephthalate) (PET) film was carried out by using wide-angle X-ray diffraction with synchrotron X-rays. Results indicated that the mesophase was developed during stretching, immediately upon necking below T-g. A sharp meridional peak was observed during the mesophase formation. The d-spacing (10.32 Angstrom) of this peak was smaller than the monomer length, indicating that the chains in the mesophase could be tilted with respect to the stretching direction. Prior to crystallization, the intensity of this peak was found to increase upon stretching. The triclinic PET crystalline structure began to form as the temperature was increased above T-g. Then, the corresponding intensity of the d = 10.32 Angstrom peak was found to decrease. This observation suggests that the triclinic PET crystals form an inclined layered structure, which shifts this peak out of the meridian. A two-dimensional analytical method was used to deconvolute the diffraction pattern into isotropic and anisotropic contributions. The isotropic and anisotropic fractions remained almost constant after stretching was stopped and during crystallization, suggesting that the strain-induced crystallization occurs mainly in the mesophase, supporting the hypothesis that the intermediate mesophase acts as a precursor for crystallization in oriented PET.