Cold drawing is known as a thermodynamically reversible process for glassy polymers. In the extension process, there might be a stage in which the global structure of chain coils extends in the drawing direction. This extension stage has not been clearly identified because of the lack of a quantitative relationship between the global structure evolution and the local structure reorientation. We have developed a theoretical method to elucidate the relationship, which is based on a simulated chain conformational distribution and a relationship between the chain conformations and the extension. Any local structure, such as the fraction of trans conformations of ethylene glycol of poly(ethylene terephthalate) (PET), can then be calculated with an ensemble average as a function of the system extension. Combined with infrared conformational. band measurements, the coil extension stage has been characterized in the cold drawing of amorphous PET. This stage does not start from the yielding point but instead begins nearly at the end of the stress softening. In the recovery process, the experimental measurements and theoretical predictions are well correlated.