The effect of physical aging on excess enthalpy has been investigated for the amorphous poly(ethylene terephthalate) (PET) glasses prepared by both means of quenching the melt into liquid nitrogen (type I) and casting the melt on a rotating metallic roller at 10 °C (type II). The relaxation toward the equilibrium of the enthalpy, monitored by a differential scanning calorimeter (DSC), is described by a relaxation fraction of the excess enthalpy φ as a function of aging time and temperature. On the physical aging in the two PET samples, a plot of φ against log tagives an essentially straight line for the PET I sample, and the plot for the PET II sample can be divided into three distinct regimes with increasing log ta: a short‐time regime where φ linearly decreases with log ta, a middle‐time regime where φ takes an almost constant value, and a long‐time regime showing a secondary decrease in φ. When the PET II is aged at 60–70 °C, two endothermic peaks in a DSC scan arise in the middle and the longer regimes of the aging time. The peak positions as well as their magnitudes considerably vary with the aging time. These results for the PET II suggest that polymer‐chain rearrangement during the physical aging process is a dissipative process which results in nonhomogeneous glassy structures consisting of low‐ and high‐enthalpy regions.