Using FTIR spectroscopy we have examined conformational changes in the quenched and slowly cooled amorphous PET films during physical aging process. It was observed that the amount of trans conformers for quenched sample decreased upon sub-T-g annealing. For the slowly cooled sample that corresponds to a state closer to equilibrium, the amount of trans conformers hardly decreased, but increased gradually during sub-T-g annealing process. The conformational populations of these two samples tend to be identical with annealing time. These results demonstrate that sub-T-g annealing will lead to closer interchain packing and result in the formation of new cohesional entanglements along the chains. In situ FTIR studies on the conformational changes of these samples were also carried out during heating up of the sample through the glass transition region. The results showed that incremental changes of the amount of traits conformers in Samples Q and SC were gradual, while an abrupt change of trans conformers occurred in the sub-T-g annealed samples. These results were in agreement with the formation of the interchain cohesional entanglement due to sub-T-g annealing.