The effects of draw ratio and ions (K⁺ and Ca²⁺) on the viscoelastic behavior of linear crystalline PET film in air, distilled water and ion so]unions were investigated by the stress relaxation methods. The Initial stress was increased with an increase of the draw ratio due to the increase of degree of crystallization and orientation, and also was Increased with concentrations of ion because of strengthening effect of ions on the hydrogen bonds and van der Waal''s frce in PET. For the overall stress relaxation behaviors of PET film, the relaxation rate of the initial stage was increased with draw ratio and ion concentrations whereas that of later stage was decreased with draw ratio and an increase of ion concentration. Montgomery''s equation and "Equilibration theory of transient phenomena" by Eyeing were applied to interpret the stress relaxation behaviour of PET film.