Pursueing high efficiency polymerization of poly (ethylene terephthalate)[PET], therlnodynamic and kinetic studies of esterification reaction between ethylene glycol (EG) and tcrephthalic acid(TPA) were performed. When a phase diagram was constructed with a function of TPA mole fraction in EG. the minimum cloud point was shown at the 30 mole % of TPA. The product at the composition contained most abundant bis- (hydroxyethyl terephthalate) [BHET] and it was verified by HPLC analysis. The phenomenon was explained with stoichimetry between TPA and EG. The effect of catalysts on the esterincation reaction was also studied by using KCA catalyst developed in KIST and Sb2O3 traditionally used in PET polymerization. In the presence of KCA catalyst the products were rather homogeneous and the catalyst acted to elevate the rate of reaction by a reduction of activation energy. The former behavior was verified by reduced number of peaks in DSC thermal diagram and the latter behavior by a reduction in clearing temperature. The elevated rate of the rcaction in the presence of KCA catalyst correlated to an increase in specific conductance originated from an increase in ionic concentration in the system.