We studied the seasonal distribution pattern of Cyanobacterium Synechococcus and heterotrophic bacteria with the epifluorescence microscope, the grazing pressure using size-classed and dilution method, and their relations with some important hydrological phenomenon on the bacterioplankton distribution, and samely discussed the potential annual variations of bacterioplankton in Southern Yellow Sea, China.1. In spring, the average abundance and biomass of Synechococcus was 3.71×10~4 cells ml~(-1) and 10.91μg Cl~(-1) respectively, while those of heterotrophic bacteria was 3.86×10~5 cells ml~(-1) and 17.72μg Cl~(-1), based on the samples collected in June 2002, 2003 and 2004. With regard to their vertical distributions, Synechococcus always showed its density in the middle water layer (10~20m) and scarce in bottom water layer. Heterotrophic bacteria showed more in the middle and surface water layer, though it had some difference in the three cruises.The distribution of bacterioplankton was influenced by the Yellow Sea Water and Subei Coastal water in surface water layer and by the Qingdao Cold Water Mass, Yellow Sea Water, Subei Coastal Water and the Changjiang (Yangtzriver) Dilution Water in bottom layer. Maximal biomass value of Synechococcus mainly occurred in the tidal front, or the middle water layer, the surface of the stratified zone. The maximal abundance of heterotrophic bacteria occurred in the surface, bottom waters in the mixed zone of different waters and the surface water in the stratified zone vertically. The tidal front might be an important factor to affect the distribution of bacterioplankton biomass in the late-spring and early-summer in the southern Yellow Sea.2. In summer, the results showed: 1) the biomass value of Synechococcus ranged in