The effect of molecular weight of polyethylene glycol(PEG) on two kinds of copolysters was investgated. One is polyethylene terephthalate polyethylene glycol(PET-PEG) block copolyester
the other is - SO3Na group contained PET-PEG copolyester. It was shown that the higher the molecular weight of PEG the quicker the crystallization rate of copolyester. The result of difference in dyeability between copolyesters with PEG of different molecular weight was also obtained.