Several methods have been proposed to increase the interfacial adhesion between the reinforcement agent and the polymer matrix, including fiber modifications and grafting maleic anhydride (MA) to the matrix as compatibilizer. Poly(trimethylene terephthalate) (PTT) is a thermoplastic polyester with several advantageous properties, such as, good tensile strength, resilience, outstanding elastic recovery and dyeability, which makes it an ideal candidate for applications in textile fiber and engineering plastic [1]. MA grafted PTT (PTT-g-MA) can be used as compatibilizer for PTT blends. This compatibilizer agent reacts with the PTT chain improving the interaction between the two phases of the blend to make the phase morphology more stable. The grafting reaction of MA to PTT chains was made by reactive processing in a twin-screw extruder at temperature profile of 230/235/235/240/240 °C from the first section to the die, operating at screw speed of 100 rpm and feeding of 30 rpm. PTT-g-MA was produced with addition of MA and benzoyl peroxide was used as initiator. The PTT-g-MA was characterized by Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and rheological behavior by parallel-plate rheometer. FTIR was used to prove the presence of MA in the PTT chains which confirmed the grafting by the formation of new peaks in spectrum. The viscosity of neat PTT and extruded PTT are nearly unchanged with increasing shear rate, belonging to the Newtonian fluid. But extruded PTT had a lower viscosity due to chain breakage during processing. At low shear rate range, the viscosity of PTT-g-MA did not change, while at high shearing rate, the viscosity decreases sharply with increasing shearing rate, belonging to a pseudoplastic fluid. This behavior is due to the disentanglements between the molecular chains. References: [1] Wang, J.; Wang, C. and Run, M. International Journal of Polymer Science, vol. 2013, Article ID 890749, 8 pages, 2013.