Using the fractional polarization thermally stimulated discharge current technique the shift of the peak position in the temperature range of the glass transition of the amorphous phase for polyethylene terephthalate is analyzed. The difference between the temperature of maximum intensity of an elemental peak T-m and the polarization temperature T-p provides information about the contribution of the molecular mobility to the dielectric relaxation. The temperature of 363 K was identified as the lower limit at which the dielectric manifestation of the glass transition becomes significant and the temperature of 403 K represents the higher limit of dielectric manifestation of the glass transition. It is proposed that the dielectric transition temperature associated with the non-local alpha relaxation is to be determined from the condition T-m = T-p. At this temperature the dipoles move equally fast during polarization or depolarization stage. The activation energy continuously grows and the pre-exponential factor significantly decreases as the polarization temperature increases. The relaxation time tau(T-m) at the temperature of maximum intensity of an elemental peak decreases from 762 to 545 s as T-m increases from 356 to 391 K, demonstrating a narrow distribution of relaxation times. A structural change has been observed around 368 K.