The effect of solvents on the facilitated transport of ethyl ester of docosahexaenoic acid (DHA-Et) was studied in the system where feed phase, membrane phase and receiving phase had the same solvent. The carrier of DKA-Et was Ag+ and Nafion(R) membrane was used as the support. First, the swelling behavior of Nafion(R) membrane was investigated in Various solvents. The solvent contents in the membrane were well correlated with the donor number (DN) of the solvents and had the maximum at DN of about 25 kcal/mol. Stability constants of complexes between Ag+ and DHA-Et were obtained from the extraction equilibrium experiments. The stability constants increased with the decrease of DN of the solvents. From the balance of two effects on the membrane swelling and the stability constants, the solvent with DN similar to 19 kcal/mol, that is aqueous methanol solvent showed the highest permeance of DHA-Et in Ag salt form membrane. When DMF, DMA and TBP, which have about 25 kcal/mol of DN, were used as the solvent, the permeance in Na salt from membrane was high due to the high membrane swelling, while the facilitation factors and the selectivities of DHA-Et to ethyl ester of oleic acid were low because the complexation ability between Ag+ and DHA-Et was poor. The heat-treatment of Nafion(R) membrane in glycerin and the increase of the experimental temperature were found to be useful to enhance the permeances because of the larger membrane swelling.