The distribution of the abundance of phytoplankton, chlorophyll a concentration and primary productivity in the northern South China Sea and the Beibu Gulf were observed in August,1999. The results showed that there were marked features of spatial zonation in the surveyed area due to the differences between the geographic environment and the hydrological conditions. Chlorophyll a value were higher near the Zhujiang (Pearl) River and lower near the eastern Hainan Island and southeastern Hong Kong at the northern South China Sea. Chlorophyll a in the surface decreased from nearshore to the mouth of the Beibu Gulf and the mean value in the water column were low at the center and mouth of the Beibu Gulf. The average concentration of chlorophyll a, primary productivity, and abundance of phytoplankton in the northern South China Sea and the Beibu Gulf were (2.03±1.18) and (0.69±0.39) mg/m3, 88.7 and 233.7 mgC/(m2·d), 0.38×106~1233.18×106 and 2.10×106~47.88×106cel/m3, respectively. The results of the size- fractionation showed that the contribution of picoplankton (<2μm) to total chlorophyll a and total primary productivity were in dominant position in phytoplankton community biomass and production of the northern South China Sea and Beibu Gulf. Nanoplankton (2-20 μm) were less, and netplankton (>20μm) the least.