Thermal ring-opening polymerization of uniform macrocyclic ethylene terephthalate dimer was examined at various temperatures above its melting point. From SEC study, it can be indicated that the polymerization proceeded through chain reaction, not through a typical polycombination reaction. Maximum molecular weight of the polymer obtained under optimum reaction conditions reached to a sufficiently high level of more than 20,000 (M-w). It is noteworthy that this new reaction for synthesizing poly(ethylene terephthalate) progresses without any catalyst under atmospheric pressure.