Poly (ethylene terephthalate) (PET) is prone to the hydrolysis and thermal degradation reactions which occur during melt processing. These reactions and linear structure limited its applications and recycling of PET. In this work, we follow the synergistic effect of using pyromellitic dianhydride (PMDA) and pentaerythritol (PENTA) as chain¬ extender and Dibutyltin dilaurate (DBTDL) for chain extension process during reactive extrusion. Rheological properties of the samples were measured by a parallel plate rheometer. The experimental results shown that by increasing amount of PMDA the complex viscosity of PET increase and formulations which contain also PMDA and PENTA shown a significant increase in this trend. Furthermore, by increasing the amount of DBTDL in the modification process an improvement was observed in rheological properties.