In connection with the danger of a fire, there have been many studies to improve the flame retardancy of polyesters. In particular, flame-retardant (FR) polyester yarns have been produced by blending or copolymerization with an FR master batch or FR additives. Nowadays, polyesters that are almost inherently FRs are produced with copolymerizable phosphorous FRs. Commercially available several types of phosphorous flame retardants for polyester were compared from the viewpoint of chemical reactivity, reaction mechanism, and byproduct formation that one of those is 3-(Hydroxyphenyl phosphinyl) propanoic acid (HPP). In this research, flame retardant poly(ethylene terephthalate) (FR PET) synthesis is studied in the presence of antimony trioxide (Sb2O3) as catalyst and HPP as flame retardant additive in a PET pilot plant. The effect of HPP on the flame retardancy of final produced FR PET has been investigated with some tests such as Limiting Oxygen Index (LOI). The results of Characterization of PET and FR PET samples have shown that the modification of PET with low levels of HPP comonomer, increased LOI about 10-15%. On the other hand, nature of phosphorous component, had excellent effect on the flame retardancy properties of final produced FR PET in compared with general PET. Also, with utilize HPP as flame retardant additive, Tm was decreased. In presence of phosphorous based FR, crystallinity percent was decreased and this subject directly affected on the melting point temperature.