The siphonaceous genus Udotea is a group of widely distributed tropical algae, although one species extend its distribution to subtropical regions. Since the monographic study of this genus by A. and E. S. Gepp in 1911, only new species ( U. sublittoralis Taylor 1928) and one new variety (U. palmetta var. marshallensis Dawson,1957) have been added to the genus. Gilbert (1962) described from Hawaii Udotea (?) abbreviate Gilb. which is, however, non-calcified and Flabbelaria in structure, and is better transferred to that genus, becoming F.abbreviata comb.nov. In April to May, 1957 and March to June 1958, the Institute of Oceanology, Academia Sinica, sent two collecting groups to the Xisha Islands of the Guangdong Province, these being coral islands in the central South China Sea, and 113 nos. of Udotea spp. were collected. A preliminary analysis of these specimens showed that the Xisha Udotea flora is very rich, and the six new species described in this paper are only a part of it. The specimens studied including the type specimen are all deposited in the harbarium of the Institute of Oceanology, Academia Sinica at Qingdao.