Abstract:Abundance of Synechococcus and picoeukaryotes around Leizhou Peninsula were estimated using flow cytometry in summer and winter of 2006. In the summer, the abundances of Synechococcus and picoeukaryotes were (5.74±5.0)×104cell/ml and（1.81±1.16）×103cell/ml respectively; and in the winter they were （1.57±2.17）×103cell/ml and （4.17±4.40）×103cell/ml.Synechococcus were much more abundant in the summer than in the winter, and more abundant in Leizhou Bay than in other areas. In contrast, picoekuaryotes were more abundant in the winter than in the summer in most locations, and reached their highest abundance in the surface waters of Leizhou Bay in winter. Seasonal variation of autotrophic picoplankton population was higher in the waters of west than east of Leizhou Peninsula. Nutrients，temperature，and tide were key factors controlled Synechococcus and picoeukaryotes population variations around Leizhou Peninsula.