The initial stage of a cold crystallization process for uniaxially stretched poly(ethylene terephthalate) (PET) was examined in detail by means of fluorescence spectroscopy and wide-angle X-ray diffraction measurements. The cold crystallization of PET annealed at around T-g, which occurred by way of mesomorphic ordering,, was investigated with respect to the pi-pi interactions between the main-chain phenylene groups of PET by measuring the polarized fluorescence intensities and the distribution of the ground-state dimer (GD) of PET. The mesomorphic ordering was found to be induced by GDs acting as nucleation centers; these GDs seemed to progressively assume a smectic form upon annealing at temperatures around T-g. Moreover, the flat plane in the smectic form was suggested as the cause of the unique tilted orientation observed in triclinic PET crystals.