Thyroid hormone, mainly active triiodothyronine (T3), plays important roles during the fish post-embryo development including metamorphosis. Because of very small amount of T3 in whole body of larva, its distribution in different tissues during fish post-embryo development is not easy to be detected. As one kind of deiodinases, deiodinase I can convert thyroxine (T4) into the active T3. The expression pattern of Dio1 gene during post-embryo development can indirectly indicate the T3 distribution in different tissues. Dio1 gene is mainly expressed in intestine, gill, liver, body muscle, fin, and skin around eyes in post-embryo developing flounder Paralichthys olivaceus, indicating T3 regulates those tissues development. Specially during the process of anal fin and dorsal fin (including crown-like larval fin) development, expression pattern of Dio1 was evidently associated with the formation of fin fold, fin rays and pterygiophores, suggesting thyroid hormone plays important role in regulating media fin development.