Abstract: Increasing evidence show that cryptic and pseudo-cryptic species are widespread in several diatom taxa. This is the case with the genus Pseudo-nitzschia, which includes chain-forming pennate diatoms that bloom both in coastal and open ocean waters. In the last years, up 10 new species have been described on the basis of morphological and molecular investigations, and some of them are only distinguishable on the basis of minute ultrastructural features of their siliceous frustulae. This is the case of P. pseudodelicatissima complex. It has been confirmed that P. pseudodelicatissima complex was a kind of pelagic diatom species which distribute commonly in Chinese coastal waters. Because of few subtle morphological features which just could be observed under electronic microscope (EM) among P. pseudodelicatissima complex taxa, it is greatly difficult to reach accurate identification and there are many debates at present. For clarifying the species diversity of P. pseudodelicatissima complex and its dominant species in Daya Bay, water and net samples were collected from six stations in summer (August), 2007. All samples were fixed with Lugo’s solution in situ and brought back to the laboratory. Then, they were concentrated to a final volume about 2—10 mL by settlement overnight for several times. These samples could be observed and analyzed under light microscope. For observation under transmission electron microscope, subsamples were taken and treated through several steps. Firstly, after dropped into same volume of concentrated H2SO4, these samples were boiled in the water bath for about 15—20 minutes to remove the organic materials. Secondly, these samples should be washed to neutrality with distilled water for several times. Then, these samples could be observed, identified, photographed and counted under transmission electron microscope. Including six species, P. caciantha Lundholm, Moestrup & Hasle, P. calliantha Lundholm, Moestrup & Hasle, P. cuspidata (Hasle) Lundholm, Moestrup & Hasle, P. mannii Amato & Montresor, P. pseudodelicatissima (Hasle) Lundholm, Hasle & Moestrup and P. sinica Qi & Wang, P. pseudodelicatissima complex were found. And P. caciantha, P. calliantha, P. mannii were identified as three newly recorded diatom species for China. Along with LM and TEM photos, the detailed morphological description, habits and ecological distribution of these six species were given in this paper. Simultaneously, the taxonomic comparison among resembling species were made. The six species described in this paper have morphological features in common: the long and slender valves, the present of a larger central interspace, the uniseriate striae, the similarity of the structure of the valve face and the valve mantle, and the hexagonal perforation pattern of the poroid hymen. However, several other features such as structure of the poroid hymen, density of fibulae and striae, the width and shape of valves, and the structure of the cingular bands differed among the species and could be used for differentiating these six species from each other. After investigation of the cell abundance of each species, it is concluded that the most common species distributing in Daya Bay is P. calliantha, not P. pseudodelicatissima which has been reported formerly.