Thermal ring-opening polymerization of a uniform macrocyclic ethylene terephthalate dimer with and without catalyst was investigated for the first time. Although polymerization progressed without a catalyst, the reaction was extremely slow and all the products were colored. Various transesterification catalysts were tested for their activity toward this ring-opening polymerization. Among the various catalysts, 1,3-dichloro-1,1,3,3-tetrabutyldistannoxane exhibited the highest catalytic activity, and a colorless polymer with a weight-average molecular weight of 36,100 was obtained in 100% yield by heating for 3 min at 200 degrees C. It is noteworthy that our method does not need a vacuum because no side products are formed during the process.