The wintering flock of the Mandarin Duck Aix galericulata was observed from January to March 1999 at Talsodo near Cheju Island, Republic of Korea. The Mandarin Ducks are globally threatened and have been estimated about 50, 000 birds all over the world. They have been designated as a natural monument, No.327, in Republic of Korea. Mandarin Ducks were observed at the study area from 19 January to 15 March. The number of Mandarin Duck showed a peak, 2, 550 birds, on 27 January. This is the first record of a large wintering flock for the species in Cheju Island, Republic of Korea. This area is an important habitat for wintering Aix galericulata. We need to have a protective policy such as monitoring the population size and disturbance factors, etc. of the habitat for the conservation of this species.