Sea-grasses are aquatic plants completely adapted to live in the marine environments.The number of species of the Xisha Islands is rather small, about 3, belonging to 2 genera, namely, Thalassia hemprichii (Ehrenb.) Aschers, Halophila ovalis (R. Br.) Hook. and Halophila beccarii Asschers. They are all first records for Chinese water. In this paper, descriptions of the three species are presented.