After its introduction as a suitable material for synthetic fibers, about fifty years ago, poly (ethylene terephthalate) (PET) has found new, interesting, and enduring applications, in many cases without any alternatives. The opportunities for PET applications are related to its properties, originating mostly from its structural characteristics. In general, the range of applications is based on the typical set of properties offered by the material, which are a result of its chemical structure and its processing history. In other words, the chemistry during material synthesis and chemical changes during post reactor processing have an influence on the desired application. In this research, comparison between two methods of increasing the molecular weight of PET-chain extension method and solid state polymerization method-separately and simultaneously has been investigated. The experiments were based on the addition of a multifunctional glycidyl compound (TGIC) as reactive additive in the melt phase and transmission of the resulted product to the solid state polymerization reactor. Finally, the effect of chain extender and solid state polymerization on the polymer properties such as molecular weight and end group concentration will be investigated. Experimental works such as inherent viscosity, end group titration, X-Ray diffraction, and FTIR have been done to study the polymer properties. The results show the significant increase in the molecular weight of samples treated in solid state phase which is more than the molecular weight of samples treated with chain extenders in the melt phase. The results show the reduction in the molecular weight of samples treated with chain extenders in the first step and solid state polymerized in the second step, due to the end group consumption by the chain extenders, in the first step which causes the lack of end groups needed for the second process. So, the condition causes thermal degradation in polymer chains and decreases the molecular weight of polymer.