The crystallinity of poly(ethylene terephthalate) (PET) films was investigated by comparing measurements of fluorescence, differential scanning calorimetry and density. It was found that the ratio of the fluorescence intensity at 330 nm to that at 370 nm can be an effective indicator of crystallinity. The dynamic process of recrystallization was directly observed for PET prepared using a spin-casting method on quartz disks by means of fluorescence measurements at several temperatures between 380 and 400 K. The recrystallization of such films was found to proceed one-dimensionally under the mechanism of heterogeneous nucleation, since the Avrami exponent, n, was determined to be one. The apparent activation energy of recrystallization was determined to be 120 kJ/mol, indicating that more than a few monomer units of PET participate in reorientation and crystallization. The present work should provide a quick and nondestructive method for determining the crystallinity of PET factory products.