The investigation of microalgal of the Xisha Islands was carried out in spring, 1978。 Twenty seven species belonging to genera of the pennate diatoms have been identified, among them 8 species are tropical neritic, appearing in the coastal regions of both the warm-temperate and tropical seas, the others are cosmopolitan. Eight species (Synedra rostrata, Podocystis adriatica, Navicula plicatula, N. pennata, N. pygmaea, Mastogloia pisciculus, M. quinquecostata, Amphora turgida )are new records for China.