Copolyesters containing sodium sulfonate group and poly(ethylene glycol) (PEG) were synthesized with various molecular weight of PEG. The selected molecular weights of PEG were 1000, 2000, 4000, 6000, and 16000. The thermal properties and the crystallization behaviors of the copolyesters were investigated by using differential scanning calorimeter (DSC). Alkaline hydrolysis of the copolyesters was studied as a function of the treatment time. The melting temperatures and the heats of fusion of the copolyesters increased with an increase in the PEG molecular weight, while the weight reduction rate of copolyesters in an alkaline solution decreased with an increase in the PEG molecular weight. At the same PEG content in copolyesters, hard segment length increases with the PEG molecular weight, which was thought to be the most important factor that determines the thermal properties and the alkaline hydrolysis behavior.