Abstract: With the consideration of all the reactions involved in the polyesterification of BHET (bis-β-hydroxyethyl terepliathalate) such as the primary forward polycondensation reaction, its reverse reaction and the cracking reaction, a kinetic equation was derived. As the concentration of ethylene glycol (EG) appeared as a term in the derived equation is difficult to determine, it was empirically correlated with the speed of stirring, R, the specific surface of the reacting system, etc. Finally a semiempirical equation correlating the average degree of polyesterification ( DP )with both the operating parameters (such as the temperature T, pressure p and the time of residence ) and the constructional parameters of the industrial reactors(such as R and ε)has been obtained as follows: