Carbon fixed by phytoplankton and cultured algae is discussed by in situ investigation and data analysis in China coastal seas. The results show that the carbon fixed by phytoplankton is about 2.22×108t a^-1 with a clear seasonal variation in the Bohai Sea, the Yellow Sea and the East China Sea, which decreases from spring, summer, autumn to winter in turns and the carbon fixed by phytoplankton in spring and summer occupies 65.3% of that in a year. The carbon fixed by phytoplankton has clear variation in October and November from 1999 to 2005 in the southern Yellow Sea., and the minimum is 3.54×104 t d^-1 and the maximum is 1.66×105 t d^-1, with an average of 1.05×105 t a^-1, but the fixed carbon has no regular variation tendency in every year. Among the influence factors for carbon fixed by phytoplankton, the phosphate concentration in seawater is the most important, and then the Chl-a and NO2-N in seawater. The carbon fixed by phytoplankton is about 4.16×108t a^-1 in the South China Sea, which is twice of that in the Bohai Sea, the Yellow Sea and the East China Sea. The seasonal variation of fixed carbon which the highest value appeared in winter and the lowest in summer is clearly different from the east China seas (including the Bohai Sea, the Yellow Sea and the East China Sea). The carbon fixed by phytoplankton in total China coastal seas is about 6.38×108t a^-1 and accounts for 5.77% of that in global margin seas. According to marine aquaculture rusults, Gracilaria lemaneiformis can fix 8.18 t carbon in a year, and the rearing density and models play an important role in its yield and carbon fixation. In recent years, the yield of the large-sized economic seaweeds in China marginal seas is about from 1.20×106 to 1.50×106 t a^-1, and it is estimated that carbon fixed is about from 3.6×105 to 4.5×105 t a^-1 with an average of 4.0×105 t a^-1. If the seaweed cultured yield increases 5% every year in China marginal seas, carbon fixed by the large-sized economic seaweeds will reach to 5.7×105 t a^-1 in 2010. Therefore, algae breeding, which has much benefit, is a very important way for increasing marine carbon sink.