We have so far known approximatly 120 species in the Genus Ceratium. Most of them have been found only in the warm seas, they are importent members in tropical phytoplankton community as well as in that of Xisha and Zhongsha regions.Their habits adapt them to difinite range, so that some speies have usualy been used as good indicators for certain sea current or water mass. During the periods of May to June of 1974 and Mareh to April of 1975, we had collected a lot of phytoplankton samples from the Xisha and Zhongsha Islands including their adjacent regions (15°28′N-17°30′N,111°10′E-114°50′E) by means of a Juday Net (its mesh size is 68 μ2), these are the materials which the present study was based on. In this paper 31 spcies of the Genus Ceratium are dealt with, of which 20 species and 4 varieties are recorded for first time from our country, and 1 variety is emended. Most of these species are familiar in the warm seas, bu tsome of them, such as Ceratium cephalotum, C incisum, C geniculatum, are stenothermal species, which are very rare in the world oceans. C. schroederi, C. fusus var. seta. C. deflexum and C.bigelowii are tropical Indo-pacific species, showing the biogeographical and marine ecological characteristics of the Xisha and Zhongsha areas.