The theological properties of polyethylene terephthalate (PET)-styrene butadiene rubber (SBR) blends are studied in this work. The shear viscosity was measured in a special capillary device mounted on an injection-molding machine, which provides data on pressure and flow rate. Comparison of the viscosity of the PET-SBR physical blend with that of the blend of PET with maleic anhydride-functionalized SBR (SBRg) provides information on the effect of grafted maleic anhydride chains at the rubber-thermoplastic interface on the theological properties of the blend. Shear viscosity is a function of the amount of compatibilizer and applied shear stress present in the preparation of the blends. A reduction in the particle size due to increasing screw speed was correlated with a reduction in the shear viscosity and an increase in the impact properties of the blend.