The marine algae collected from Lan-Yu (Orchid Island) were identified and listed. A total of 159 specie s of seaweed, including 3 species of Cyanophyta, 59 of Chlorophyta, 22 of Heterokontophyta, and 75 of Rhodophyta are reported here. Among them, 50 species are recorded for the first time in Lan-yu, most of algae in Lan-Yu are of tropical or subtropical elements. The effect s of current on the distribution of algae and the floristic affinity of this island are also discussed.