The selective transport of the ethyl ester of docosahexaenoic acid (DHA-Et) was studied in the facilitated transport system where feed phase, membrane phase, and receiving phase had the same ethanoi diluent. The carrier of DHA-Et was Ag+, and a Nafion membrane was used as the support so that Ag was immobilized in the support by electrostatic force. DHA-Et was sufficiently transported in this system due to the swelling of Nation by ethanol, while the methyl eater of linoleic acid was hardly transported in a Nafion membrane containing aqueous Ag+ solution (membrane solution). DHA-Et, which has six carbon-carbon double bonds, was transported fairly faster than the ethyl ester of oleic acid which has one double bond with a selectivity of about 10. This indicated that our membrane is useful for separating esters of polyunsaturated fatty acids based on the difference in the double-bond numbers. The effects of the water content in the ethanol solution used as diluent and the feed phase DHA-Et concentration on membrane performances were investigated. This type of facilitated transport membrane was stable for more than 200 days. This is because in addition to the immobilization of Ag+ in the support membrane by the electrostatic force, the membrane phase had the same diluent as the feed and receiving phases so that leakage of the membrane solvent did not occur.