The seaweed, Caulerpa serrulata were incubated to investigate the effects of increased atmospheric CO₂ and N supply in seawater on the seaweed. Results indicate that either increased atmospheric CO₂ or N supply can enhance the relative growth rate. The relative growth rate of C₊N₊ was higher than that of C₀N₀ group by 63.77%.In addition, Increased CO₂ decreased the contents of pigments, soluble protein and carbohydrates, and the contents of chlorophyll a in C₊N₀ was 39.01% lower than that in C₀N₀ group. C₊N₊ group was lower than those of C₀N₊ group by 49.26%, and corresponding Car in high carbon group decreased by 47.62% and 43.24%. Increased atmospheric CO₂ reinforces the activity of nitrate reductase under the conditions of both enriched and unenriched N supply, and high N supply stimulates the uptake of N and P.