Polyethylene terephthalate (PET) is one of the most important thermoplastic materials with wide field of applications. The production of PET items such as fibers, films and bottles for food and household packaging involves many processing steps. These steps result in changes of morphological and structural characteristics. Increase or decrease in the crystallinity caused changes in the physical properties of the end product. Shrinkage, dimension stability, stiffness, dye-ability, mechanical behaviour and gas barrier properties of the final products are related to the degree of crystallinity and the glass transition temperature of PET. Chemical modification via copolymerization is one of the most attractive techniques in order to differentiate the properties of PET. Hot filling procedure used for the production of shelf-stable juice involve pouring the heat-treated juice, without significant cooling, directly into the package. Usually, the temperature for hot filling of juice in PET is 80 to 88°C. In this research, synthesis of poly(ethylene terephthalate-co-naphthalene) (PETN) copolymer was studied in presence of different amounts of 2,6- naphthalene dicarboxylic acid (NDA) to Achievement the Hot Fill Properties. The microstructure of the synthesized polymer was studied with Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR). Differential scanning calorimetry (DSC) was used to study the thermal properties. The DSC results have shown that the modification of PET with low levels of naphthalene comonomer (5-8 mol%) increases the Tg. This property has a significant effect on the thermal stability potential of many hot applications to resist higher temperatures without shrinkage. Based on the FTIR and NMR results, the synthesized copolymer was Poly(ethylene terephthalate-co-2,6- naphthalene dicarboxylic acid). Results showed that PETN was suitable for use in the manufacture of hot fill containers.