Conformational conversion and molecular dynamics of the amorphous poly(ethylene terephthalate) (PET)annealed above T(g) were investigated by 13C high-resolution solid-state NMR and differential scanning calorimetry (DSC). Rapid increase in the PET trans content and crystallinity were observed near the onset crystallization temperature (≃120°C). By comparing both changes in conformation and crystallinity during thermal annealing, we found that the trans conformer increases continuously while the degree of crystallinity levels off during the later stage of annealing. The 1H spin- lattice relaxation time in the rotating frame (T(1ρ)/(H)) correlates linearly with PET crystallinity but less well with the trans content. A three-domain model containing crystalline (all trans), constrained noncrystalline (trans rich) and amorphous (gauche rich) phases satisfactorily explains the relationship among the PET conformation, crystallinity and annealing conditions. This study complements the prior study of molecular motion by T(1ρ)/(C) and illustrates the importance of the trans conformer to induce PET crystallization.