The abundance and distribution characteristics of Synechococcus in the East China Sea(26°~30°N, 120°~127.5°E)in summer were investigated by epifluorescence microscopy. The average abundances of Synechococcus and CB/PB were 1.65×10⁴ cells/mL and 4.85μgC/L respectively.The abundance of Synechococcus decreased from coastal waters to external waters in horizontal direction. In vertical distribution the abundance of Synechococcus shows obvious difference among four depths, and decreases with depth from surface to bottom.The average value of CB/PB was 22.65% in the investigated area. In horizontal direction,the CB/PB increased from coastal waters to external waters. Pearson correlation analysis between Synechococcus or CB/PB on the one hand,Chl.a, water temperature, salinity, turbidity on the other,showed that there was a positive correlation between Synechococcus and Chl.a and water temperature, while there was a negative correlation between CB/PB and NO₃^--N, TIN, PO₄^3--P, salinity and TU.