Abstract: Skeletonema costatum and Prorocentrum dentatum are the two common HAB species in East China Sea, and Prorocentrum is more responsible for most HAB events in Spring since late 90's. The HAB caused by Skeletonema usually occurs from June to August. However, these two species will not bloom at the same time. Two mesocosm experiments were conducted in East China Sea in May 1998 and May 2002. In the experiment of May 1998, the cell number of Prorocentrum dentatum and Skeletonema costatum accounted for about 85% and 11% of total cell number of phytoplankton,respectively at the beginning of the experiment. In control mesocosm, Prorocentrum dentatum dominated the community during the whole experiment. In phosphate-enriched mesocosm, however, Skeletonema costatum showed a high competition capability. In May 2002, there were 4 bags in the mesocosm experiment. At the beginning of the experiment, the cell number of Prorocentrum dentatum accounted for 55～66 % of the total,and that of Skeletonema costatum accounted for 32～42 %. During the experiment, the growth of Skeletonema costatum was faster than that of Prorocentrum dentatum.On the 2nd day, the cell number of Skeletonema costatum exceeded that of Prorocentrum dentatum. It showed that under high nutrients conditions,Skeletonema costatum grew better than Prorocentrum dentatum,but under nutrients-limited conditions,Prorocentrum dentatum could survival better than Skeletonema costatum.