Abstract: Nostoc sphaeroides is an edible nitrogen-fixing cyanobacteria in paddy fields with high economic values. However, little is known about its physiological response to CO₂ and phosphorus which are essential for the growth of N. sphaeroides. In this study, we cultured N. sphaeroides at two levels of pCO₂ (400 and 2000 ppm) and different phosphorus concentrations (0.088—0.350 mmol/L) to investigate the interactive effects of CO₂ and phosphorus on the relative growth rate, pigment content, photochemical activity of PSII and photosynthetic rate. The results showed that individual CO₂ or phosphorus concentrations had significant effects on the relative growth rate (RGR), particle size and mass, light-saturated photosynthetic rate (P_m), dark respiration rate (R_d) and photosynthetic efficiency (α) of N. sphaeroides. Furthermore, CO₂ and phosphorus had obvious interaction on its particle size and mass, chlorophyll a content, dark respiration rate and photosynthetic efficiency. High CO₂ concentration significantly increased the effect of phosphorus on the particle size and mass, dark respiration rate and photosynthetic efficiency, and decreased the inhibitory effect of high phosphorus concentrations on chlorophyll a synthesis. In addition, the interactions of CO₂ and phosphorus on the relative growth rate, phycobiliproteins content, light-saturated photosynthetic rate, the maximum photochemical efficiency (F_v/F_m) and actual photochemical efficiency (Yield) were not significant. The results suggest that elevated CO₂ or high phosphorus concentrations promoted the growth of N. sphaeroides mainly by enhancing photosynthetic rate and photosynthetic efficiency. The interaction between CO₂ and phosphorus concentrations indicated that high CO₂ concentration may increase the utilization efficiency of phosphorus, and reduced the inhibition of chlorophyll a synthesis by high phosphorus concentration to improve photosynthetic efficiency.