Abstract: The marine macroalgae Hizikia fusiformis (Harvey) Okamura (Sargassaceae, Phaeophyta) was cultured under the condition of elevated atmospheric CO₂and increased N supply to investigate the effects of increased CO₂concentration and N supply on the growth, photosynthesis, and other related physiological and biochemical traits in this species. The experiment was designed for two CO₂ levels which was 350 and 700 mol · mol^-1, and two NO₃^-levels which was added 0, 500 mol · L^-1 NaNO₃in seawater. The algae cultured under non-N-enriched seawater, elevated CO₂concentration had no significant effects on growth and biochemical traits. However, the algae cultured under N-enriched seawater, elevated CO₂could effectively depress the growth.The ratio of fresh weight (FW)to dry weight (DW), the concentrations of soluble protein and soluble carbohydrate were decreased 4.5%, 15% and 32%. It suggested that increased N supply led to the accumulation of NO₂^-, and elevated CO₂concentration caused pH value to decrease. This condition was well to form HNO₂, which is toxic effect on H.fusiformis.