Polyethylene terephtahalate has been used in several areas such as fiber, film, bottle, and disposable products. Production of PET has been rapidly increasing these days. Since PET is a semi-permanent material, it has a non-biodegradable character in itself. Wasted PET products can cause serious environmental problems. Many countries around the world impose environmental legal restrictions over their abandonments. Many researches on the enviromental influence factors and treatment techniques of the wasted PET have been carried out. The main objective of this study is to develop a new sizing agent using recycling PET and improve its internal sizing effect. Dried powder of PET was used to make the modified PET. After extracting water-dispersible PET by subcritical hydrolysis, polyester resins have been extracted and triphenyl phosphate(TPP) has been added to obtain optimal internal sizing agent. It was found that the optimum dosage of TPP was 2% (per PET weight) and the hydrolysis temperature was independent on making the modified PET.