A mathematical model for continuous direct esterification reaction between terephthalic acid and ethylene glycol was proposed and verified by commercial plant data. The effect of various operating variables, e.g., reaction pressure, temperature, feeding molar ratio, and residence time on the degree of esterification, degree of polymerization, side product concentration, and liquid fraction was investigated. The simulated results shows that productivity is very sensitive to the feed mole ratio and temperature. The degree of esterification and liquid fraction increase with decreasing reaction pressure. The control of residence time and transportation time between reactors is very important to the diethylene glycol concentration since side product formation is significantly influenced by residence time. Solid phase vanishes independent of reaction condition when degree of esterifiaction is over 90 percent.？