The mass transport and the evolution of related phase transformation in poly(ethylene terephthalate) (PET) were studied.-Solvent-induced crystallization, which occurs during the transport process in PET, is mainly explained as the result of plasticization. The degree of plasticization depends on the concentration of solvent. Therefore, the behavior of crystallization can be closely related to the mechanism of mass transport. The evolution of transformation was studied by small-angle X-ray scattering (SAXS), Fourier transform infrared (FTIR) spectroscopy, optical microscopy (OM), and the density gradient column. During the process of transport, the Fickian mechanism dominated during the first stage of crystallization, and swelling was found to be responsible for the occurrence of most crystallization in the second stage. The first two processes can be classified as the "primary crystallization". The last stage is associated with the "secondary crystallization".