Characters of amorphous phase in semicrystalline, poly(ethylene terephthalate) (PET) were investigated systematically via dynamic mechanical thermal analysis (DMTA) in this paper. It was found that the storage modulus (F) and the glass transition temperature (T-g) of semicrystalline PET changed with the degree of crystallinity (X-c). T-g showed good linearity with X-c. However, neither reduction of E' in the T-g region (DeltalgE') nor loss tangent (tandelta) at the T-g presented linearity with X-c, which suggests that the two-phase model was not suitable for semicrystalline PET. It was also confirmed that the physical aging reduced the chain segmental mobility, producing higher T-g.