The use of plastics and synthetic polymers has grown dramatically in recent decades, and waste disposal of these engineering materials has become one of the major global environmental issues. Polyethylene terephthalate (PET), with an annual growth rate of ten percent in 2012, is one of the plastic materials that are mostly used in various industries. This material is produced in different grades, and is mainly used in food packaging, beverage bottles, medical engineering, and textile industries. The use of this plastic has an important environmental problem because it is not biodegradable and its environmental degradation will take several hundred years. Hence, currently, as other plastic wastes, PET wastes are burned or are land filled. However, because of environmental concerns and increased restrictive environmental regulations, it is essential to explore alternative and suitable methods for PET waste reuse. One of the viable methods for fulfilling these needs is pyrolysis. Pyrolysis is a thermo-chemical process in which large molecules are cracked by heating organic materials in absence of oxygen. Using this process for waste PET disposal leads to less pollutant release into the environment compared to disposal of PET via other commonly practiced methods (landfill, incineration, and hydrolysis). In pyrolysis process, various operating conditions such as temperature, heating rate, residence time, use of catalysts, use of additives, and operating pressure affect the distribution of its products. Among the said process variables, temperature plays an important role in the final products of pyrolysis. The aim of this research was studying the pyrolysis of textile grade PET using heavy paraffin in order to increase the liquid products of the pyrolysis. Textile grade PET (provided by Isfahan Polyacryl Company) along with heavy paraffin (purchased from Merck) were used in a home-made pyrolytic reactor under sub atmospheric conditions. The pyrolysis was carried out in presence of nitrogen gas, and the variations of temperature and mass fraction of heavy paraffin for the highest liquid yield were studied. The results showed that at 380 ? C and a mass fraction of 15 % wt of heavy paraffin, the liquid product was a maximum, and the liquid yield was increased by 5.3% wt. The mass balances were closed to more than 99 percent. Addition of heavy paraffin did not change the gas and solid products, appreciably. Analysis of liquid products of hydrolysis of PET using gas chromatography-mass spectroscopy (GC-MS) showed that the major components were benzoic acid and phenol. These two components at 380 ? C were about 80 % wt of the liquid products of pyrolysis. The addition of heavy paraffin considerably changed the composition and the number of components of the liquid products, and 15 new compounds were generated, and identified. Finally, addition of heavy paraffin reduced benzoic acid by 16.62% wt, and increased phenol by about 8% wt. Keywords : Pyrolysis, Polyethylene terephthalate, Paraffin, Thermal decomposition, Plastic recycling