The mechanism of mass transport and the related crystallization in poly(ethylene terephthalate) (PET) were studied. Solvent-induced crystallization can occur during the transport process in PET at low temperature. The important effect of changing the surrounding medium to solvent molecules is to reduce the glass transition temperature. This phenomenon is called "plasticization". The extent of plasticization relies on the amount of solvent around the polymer molecules, i.e., the concentration of solvent which will depend on the mass transport before saturation. Evidences for the transport mechanism in the first stage of crystallization were revealed. The distinct diffusion front was determined from the measurements of optical microscope and microhardness. The differential scanning calorimeter (DSC) curves displayed crystallization exothermic peaks whose areas decreased with the amounts of amorphous regions, representing the solvent-induced crystallization (SINC) process. The observed phenomenon of multiple-stage crystallizations is associated with the variation of boundary conditions during mass transport, which was ignored in most theoretical analyses.