A study has been carried out to see the effcet of solvent sorption on the mechanical properties of amorphous copolyesters. Sorption and desorption curves indicated that, among the solvents investigated, N,N-dimethylformamide(DMFA) was sorbed greatly by the copolyesters. The effect of solvent environments on the mechanical properties of amorphous copolyesters have been studied by means of the tensile test. From the mechanical behaviour of copolyesters before and after sorption, we found that change of the mechanical properties of copolyesters is explained on the basis of the plasticization, solvent induced crystallization, surface cavitation,and chemical attack.