The polycondensation of bis (2-hydroxyethyl) terephthalate has been kinetically investigated in the presence of zinc acetate as a catalyst at 26$0^{circ}C$ and 27$0^{circ}C$. On the basis of the titrated results of carboxyl end groups, a thermal degradation reaction of chain scission was assumed to occur simultaneously with the polycondensation reaction a certain propagation reaction period later. If kp and kd are the rate constants of propagation and degradation reactions, respectively, and t is the reaction time, the following relations were finally obtained for the number-average degree of polymerization, where tc is initiation time of chain scission reaction and a is a constant, $leq$1. The equations were found to be in good agreement with the experimental results.