This research investigated the seasonal and spatial distributions of picophytoplankton, including Prochlorococcus, Synechococcus and picoeukaryotes, in the northern South China Sea. Monitoring experiments including light intensity control and nutrient enrichment were conducted concurrently with on board sampling to examine factors affecting their cell densities dynamics. Quantification of cell numbers was carried out by flow cytometry. Averaged Synechococcus abundance in the South China Sea was 1¡Ñ104 cells ml-1, high in winter (1.37¡Ó0.30¡Ñ104 cells ml-1) and low in summer or fall (0.51¡Ó0.13¡Ñ104 cells ml-1 and 0.53¡Ó0.22¡Ñ104 cells ml-1, respectively). During a same season of the year, there was more Synechococcus in the shelf-slope region than in the basin. The cell density in summer, but not in winter, was significantly positively related to surface water nutrient concentration. Nutrient enrichment experiment carried out in winter also indicated that the growth of Synechococcus did not respond to addition of nitrate. On the other hand, Synechococcus seemed to prefer high illumination. In the light intensity experiment, Synechococcus collected from surface water grew better at 100% surface illumination than <100% light intensities. Synechococcus collected from deep water grew best at 30% and 18% of surface illuminations. Vertically, Synechococcus concentrated mostly in surface water with maximum cell number occurring at the surface or a few meters deep. Nutrient enrichment experiment in winter also showed that Synechococcus responded significantly to iron addition. Average cell density of picoeukaryotes was always less than 0.5¡Ñ104 cells ml-1, being high in winter (0.46¡Ó0.10¡Ñ104 cells ml-1) and low in summer or fall (0.15¡Ó0.02¡Ñ104 cells ml-1 and 0.19¡Ó0.03¡Ñ104 cells ml-1, respectively). Picoeukaryotes was always more concentrated in the shelf-slope region than in the basin, especially in winter when cell density in the shelf-slope region was 0.70¡Ó0.11¡Ñ104 cells ml-1. Although in winter picoeukaryotes was significantly positively related to surface water nutrient concentration, enrichments of nitrate or iron did not enhance their growth. Prochlorococcus had a cell density > 5.5¡Ñ104 cells ml-1 in the euphotic zone, and distributed as deep as 200 m. Light intensity monitoring experiment showed that Prochlorococcus from surface water grew better under high illumination than those from deep water and vice versa. Under 9% of surface illumination, deepwater Prochlorococcus population showed a positive growth, corresponding well with its deep distribution. Nutrient enrichment experiment conducted in winter showed that Prochlorococcus did not respond to enrichment of nitrate or iron.