Six kinds of new aliphatic ester diols were synthesized by the ring opening reaition of glycolide (or lactide) with aliphatic dials such as ethylene glycol (EG), 1,4-butanediol(BD) and 1,6-hexanediol (HD), as initiators, in the presence of stannous octoate catalyst. It was found that the prepared aliphatic ester viols coexisted on two conformational isomers of linear and ring forms in a solvent. The ratio of the concentration of ring form to linear one in the diol compounds increased in order of 0/100 (EG), 25/75 (BD), and 49/51 (HD) as the length of the methylene linkage of the diol used as an initiator increased. The viols were polymerized with conventional aliphatic dicarboxylic acids, such as succinic acid, adipic acid and sllberic acid, or the corresponding acid chlorides to produce new sequentially ordered aliphatic polyesters. The resulting polymers had inherent viscosity in the range of 0.1-0.4 dL/g in chloroform at 25 ℃ According to the results of DSC, their glass transition temperatures (T_s ) rarlged from -35 to 10 ℃. From TGA thermograms, the sequentially ordered aliphatic polyesters were observed to show relatively good thermal stability. The temperature of 10% weight loss was in the range of 270∼345 ℃ and all the polymers showed char yields of 5∼10% at 550 ℃ under a nitrogen atmosphere. The higher the length of the methylene linkage incorporated into the diol increased, the lower the T_s decreased and the better the thermal stability showed.