Abstract: PET was modified by adding WSPET.Rheological properties in extensional flow of PET,WSPET and their blend were researched by capillary rheometer.FT-IR and XRD were used to examine the structure of the polyblend fiber.The experimental results demonstrated that rheological property of all the samples researched appears tension-thinning behavior.With the increase of extensional stress,apparent extensional viscosity decreases.Extensional stress increases and apparent extensional viscosity falls as extension strain rate increases.IR and XRD studies showed that when WSPET is added,the structure of PET will not change obviously, while the crystal behavior of the polyblend will decrease.