This is the first part of a series papers dealing with the marine blue-green algae of the Xisha Islands-a group of South China Sea coral islands. The materials for study are chiefly based on the collections made by the Institute of Oceanology,Academia Sinica in 1975 and 1976. The present paper deals with 11 species of the marine blue-green algae belonging to 6 genera and 1 family, and they are all recorded for the first time from Xisha Islands, although among them Phormidium tenue had also been recorded as a subaerial blue-green algae collected from Yongxing Island in 1948. and four species (with asterish) are recorded for the first time from China. Most of the species on the Xisha Islands are cosmopolitan or widespread species.