It was predicted that the atmospheric CO₂ concentrations in the end of this century would be twice as much as the present level, and as a consequence of this the mean global temperature would elevate 4-5 ℃. At present, there are many researches on seaweeds in response to elevated atmospheric CO₂ concentrations or temperature alone. However, the investigations concerning the impacts of combined effects of elevated atmospheric CO₂ concentrations and temperature on seaweeds is very limited. The marine red macroalga Gracilaria lemaneiformis has been cultivated on large scales in both the southern and the northern parts of China. It is essential to evaluate how the climate change (such as the elevated atmospheric CO₂ concentrations and global warming) affect this economically important species. In this study, G. lemaneiformis was cultured under the following four different conditions: 1) ambient control (390 μL/L CO₂ + 20 ℃); 2) elevated CO₂ (700 μL/L CO₂ + 20 ℃); 3) elevated temperature (390 μL/L CO₂ + 24 ℃); and 4) greenhouse effect (700 μL/L CO₂ + 24 ℃). After cultured for 10 d, the growth and biochemical compositions were examined. At the same time, the changes of maximum photochemical quantum yield (F_v/F_m), light use efficiencies (α), net photosynthetic rate (P_n) and dark respiratory rate (R_d) under high-temperature stresses (32 ℃, 36 ℃ and 40 ℃) were explored. The results showed that elevated CO₂, elevated temperature, or greenhouse effect all enhanced the growth of G. lemaneiformis, with the highest relative growth rate occurring under the culture treatment with greenhouse effect. The growth condition treated with greenhouse effect increased the rates of P_n and R_d in situ, but decreased the contents of solution protein (SP) and soluble carbohydrate (SC) in algal thalli. Elevated CO₂ in culture increased the rate of P_n in situ, but the growth condition treated with elevated temperature had hardly affected the P_n in situ. Both chlorophyll a (Chl a) and carotenoid (Car) were increased with elevated temperature in culture, but their contents were unaltered with high CO₂. Elevated CO₂ or elevated temperature alone had no significant effects on the contents of phycoerythrin (PE) and phycocyanin (PC) of the algal thalli. The changes of F_v/F_m and α of the algal thalli under high-temperature stresses displayed the same tendency, i.e: their values all increased slightly under 32 ℃-stress, but decreased under 36 ℃-stress, and declined fiercely under 40 ℃-stress. In the course of 6 h of 32 ℃-stress, the rates of P_n in elevated temperature-grown algae and greenhouse effect-grown algae were much higher than those in the algae grown under control condition. In the course of 6 h of 36 ℃-stress, the rates of P_n in greenhouse effect-grown algae displayed the highest levels relative to the algae grown with other three treatments. It was shown that the high-temperature tolerance limit of photosynthesis in 20 ℃ grown algae was between 32 ℃ and 36 ℃, while that of 24 ℃-grown algae was between 36 ℃ and 40 ℃. Taken together, our results suggested that growth of G. lemaneiformis would benefit from elevated CO₂ and/or elevated temperature. Moreover, the greenhouse effect (combined with elevated CO₂ and elevated temperature) would improve the photosynthetic thermal tolerance to high temperature for G. lemaneiformis.