To obtain the adaptive capacity of macroalgae to light-change environment in future elevated CO₂ condition, we selected the economic red macroalga Gracilaria lemaneiformis as the experimental material to study the co-effects of light and CO₂ on the growth rate, respiration rate, net photosynthetic rate, chlorophyll fluorescence parameters and the contents of photosynthetic pigments. Two CO₂ levels and two light intensities were set in this study. The different CO₂ concentrations treatments were set as (1) actual atmospheric CO₂ concentration: average of the pCO₂ was about 390 μL/L; and (2) High CO₂ level: pCO₂ concentration was about 1000 μL/L. Target pCO₂ in this study was achieved by bubbling pre-mixed air-CO₂ mixtures from the plant growth CO₂ chamber, which controls the CO₂ level with a variation of less than 3%. The different light levels were attained with cool white fluorescent tubes (300 μmol m^-2 s^-1, HL; and 100μmol m^-2 s^-1, LL). The culture medium was renewed every two days using filtrated seawater and about 64 and 8 μmol/L concentrations of nitrate and phosphate were added, respectively. The culturing densities of G. lemaneiformis were controlled exactly to maintain the pH in the medium changed less than 0.05. Our results showed that the significant interactions of CO₂ and light were found in the photosynthetic performance and relative growth rate of G. lemaneiformis. elevated CO₂ concentration had no significant effect on the relative growth rate of G. lemaneiformis, but light played different roles when the thalli was cultured in different CO₂ levels. In atmospherical CO₂ condition, no difference was found between high and low light treatments; while the higher growth rate was found in the high light treatment when the thalli was cultured under high CO₂ level. CO₂ stimulated the respiration rate of G. lemaneiformis cultured under high light level, but no effect was found in that under low light condition. For net photosynthetic rate, CO₂ showed the negative effects when the thalli grown in low light level, while no effects was found in that under high light condition. The relative electron transport rates of G. lemaneiformis were not changed under different CO₂ levels, but significant inhibition was found when it was measured under high light. The lowest concentrations of Chl a and phycoerythrin were found in the thalli cultured under high CO₂ and high light condition together. From the results we get above, we can get the conclusion that increased availability of pCO₂ down-regulated CO₂ concentration mechanism (CCM), which together with the increased seawater acidity led to higher sensitivity to excessive or stressful light intensity, leading to doubled edged effects of G. lemaneiformis. We should fully consider the co-effects of other environmental factors with CO₂ when estimated the effects of CO₂ on the macroalgae.