Abstract: According to two surveys conducted in August 2006 and January 2007, a total of 182 phytoplankton samples were obtained at 91 survey sites across 15 sections. Phytoplankton samples were identified and analyzed based on morphological classification methods. It was discovered that the Trichodesmium in East China Sea consisted of Trichodesmium thiebautii,T. erythraeum and T. hildebrandii, among which T. thiebautiiwas the dominant species. In summer and winter, the distribution patterns of Trichodesmium spp. were quite similar and the densities were high in the offshore waters and in the Kuroshio waters compared with other areas. Temperature and nutrients were important controlling factors for the distribution.