Amorphous phase in semicrystalline poly(ethylene terephthalate) (PET) was investigated via its physical aging behavior. It was found that when the degree of crystallinity (f(c)(w)) was within a definite range, there were dual amorphous regions with quite different characters in semicrystalline PET: free amorphous region and constrained amorphous region. Both lower and upper limits of the f(c)(w) range increased with increasing crystallization temperature (T,) for semicrystalline samples crystallized from the glassy state. The dual amorphous regions showed as dual tan 6 loss peaks in dynamic mechanical thermal analysis (DMTA) curves, and when the samples had been physically aged, there were dual endothermic physical aging peaks appearing in the glass transition region of differential scanning calorimetry (DSC) traces. With the proceeding of physical aging, some of the free amorphous region evolved into the constrained amorphous region due to the reduced mobility of chain segments produced by physical aging. The whole amount of the amorphous phase, however, was kept almost constant during the whole process of physical aging. Amorphous phase in semicrystalline samples crystallized from the melt was also discussed.