Studies on Synechococcus and Prochlorococcus growth and mortality rates using selective inhibitor technique for the first time were conducted in the northern South China Sea in 2004. The results showed that high abundance of Synechococcus appeared in the subsurface layer (15 m or 20 m), above the thermochline. The high Synechococcus growth rate always occurred above the layer of its maximum abundance. Prochlorococcus abundance showed subsurface maximum in the summer according to the thermochline depth. Prochlorococcus growth rates in the surface were higher than that in the euphotic bottom. The range of Synechococcus euphotic-averaged growth and mortality rates were 0.11～1.18d-1 and 0.11～0.76d-1, respectively. Both distribution patterns increased from the coast to the open ocean. The range of Prochlorococcus euphotic-averaged growth rates were 0.23～0.49d-1. The rates showed obviously seasonal variation, i.e., slightly higher in coastal water than offshore in the summer and contrary trends in the winter. The range of Prochlorococcus euphotic-averaged mortality rates were 0.12～0.33d-1. The rates were higher in near shore than offshore in both summer and winter. Temperature, nutrients and light were important factors to influence Synechococcus growth rates. There were no relationship between abundances and community growth rates of Synechococcus and Prochlorococcus, but obvious exponential correlation with the mortality rates. Synechococcus and Prochlorococcus production and their contributions to Pico-production were estimated based on their growth rates.