This paper contains the results obtained by ecological investigation of flora in Jak-yak Island of Incheon Harbor in the west coast in Korea. Twenty-four varieties, 182 species, 147 genera and 63 families of Tracheophyta were recorded by field investigations conducted over two times in August, 1980(1st) and August, 1981 (2nd) and September, 1981 (3rd). The flora of this island showed a plant community predominated by Pinus thunbergii grown in mingle with Pinus densiflora, Robinia pseudo-acacia, Prunus yedoensis, Prunus Sangentii, Pueraria thunbergiana. Generally, the number of plant species was scarce, but it was an unexpected result that such mountainous plants as Convallaria keiskei, Arisaema amurense var. serratum, Aconitum pucherrimum, Polygonatum odoratum var. pluriflorum, Pseudostellaria palibiniana etc. were the undergrowth of the acacia forests (Robinia pseudo-acacia). Phyllostachys nigra var. henonis was grown in south side from the island, while no evergreen broad-leaved trees.