Abstract: Thalassiosira Cleve is a very common diatom genus with high species diversity in coastal waters around the world. Up to now, nearly one hundred and fifty taxa belonging to this genus have already been reported. Thalassiosira is the main contributor to nano-phytoplankton assemblage because cell sizes of most species are below 20 m. Previously, some Thalassiosira species were considered as model organisms of centric diatoms and Bacillariophyta, and many studies concerning genetics, sexual reproduction, feeding pressure, impact on egg producing and hatching of zooplankton, environmental indicator, etc, have been performed. On the other hand, some Thalassiosira species are also important bloom-causing organisms. And the blooms induced by T. curviseriata and T. diporocyclus have already been reported in the Changjiang River Estuary and along Hainan coast respectively. Due to their very small cell sizes, electron microscopy is needed for accurate identification. In order to characterize the species diversity and distribution of the genus Thalassiosira along Chinese coastal waters, water samples were collected from some locations. Thalassiosira specimens within these samples, as well as several clonal strains isolated from natural samples, were studied by light microscopy (LM) and scanning electron microscopy (SEM). All samples were fixed with Lugos solution and were concentrated to a final volume about 2-10 mL by settling overnight several times. The cultivated samples could be observed and analyzed under a light microscope. For observation under scanning electron microscope, sub-samples were taken and treated in several steps. Firstly, after dropped into the same volume of concentrated H2SO4, these samples were boiled in the water bath for about 10-15 minutes to remove the organic materials. Secondly, these samples were washed to neutrality with distilled water for several times. Then, these samples were coated with gold and were subsequently observed, identified and photographed under a scanning electron microscope. Three Thalassiosira species were newly recorded species for China, i.e., T. andamanica Gedde, T. constricta Gaarder and T. duostra Pienaar. Along with LM and SEM photos, detailed morphological description, habits and ecological distribution of these species were given in this paper. Simultaneously, taxonomic comparison among resembling species were made. T. andamanica was very common in South China Sea coast, especially in Hong Kong waters. The morphological features of this species were represented by prominent external tubes of all fultoportulae and one rimoportula keeping a distance from the valve margin. For T. duostra, more patterns of valve fultoportulae and three marginal rimoportulae have been found, which were different from previous descriptions.