Atmospheric CO₂ concentration is predicted to nearly double by the end of this century. There are a large number of reports on the effects of elevated atmospheric CO₂ concentrations on seaweeds. However, the investigation concerning the impacts of combined effects of elevated atmospheric CO₂ concentrations and temperature on seaweeds is very limited. In the present study, the red seaweed Porphyra haitanensis (Bangiales, Rhodophyta) was cultured under four different conditions: (1) 15℃+390 μ mol/mol CO₂,(2) 15℃+700 μ mol/mol CO₂,(3) 25℃+390 μ mol/mol CO₂,(4) 25℃+700 μ mol/mol CO₂, in order to examine the combined effects of the elevated CO₂ concentrations and temperature on growth, some biochemical components and chlorophyll fluorescence in this commercially important species. The results indicated that the growth response of P. haitanensis to elevated CO₂ in seawater was temperature dependent, with the enhancement of growth resulting from elevated CO₂ being remarkable greater at lower growth temperature (15℃) with respect to 25℃. The CO₂ conditions in culture exerted a much pronounced influence on the contents of Chlorophyll a (Chl a) and Carotenoid (Car) than temperature did. 15℃-grown P. haitanensis exhibited much higher maximum relative electron transport rate (rETR_max) in comparison with 25℃-grown thalli, regardless of the CO₂ concentrations in culture. This indicated that P. haitanensis displayed a higher photosynthetic potential at low growth temperature than high growth temperature. Meanwhile, extra CO₂ in the culture had no significant impact on rETR_max in P. haitanensis. The characters of chlorophyll fluorescence were determined under different measurement temperatures using Junior-PAM. The increase of measurement temperature from 10 to 30℃ made a negligible effect on rETR_max, the photosynthetic light use efficiency of electron transport (α, the initial slope of the rapid light curves) and maximum photochemical quantum yield values (F_v/F_m), regardless of the growth conditions. However, the values of rETR_max、α and F_v/F_m were reduced drastically when the measurement temperatures were above 30℃, with the decline being much pronounced in 15℃-grown algae than 25℃-grown algae. Our results suggested that high temperatures (30-40℃) in the short-term inhibited the photosynthetic capacity of P. haitanensis, which might be associated with the damage in photosystem Ⅱ (PSⅡ) reaction centre and a concomitant reduction of electrons transport rate. Meanwhile, 25℃-grown algae showed a higher tolerance to high temperature stress than 15℃-grown algae did, since 15℃-grown algae displayed greater downtrend in the values of rETR_max、α and F_v/F_m when the measurement temperatures rose to 30℃. The results also showed that the value of rETR_max was lower in P. haitanensis thalli grown under CO₂ enriched air with respect to non-enriched air when the algae were subjected to the high temperatures, indicating that, compared to normal CO₂ growth conditions, elevated CO₂ growth conditions might repress the ability of photosynthetic electron transport of algae when subjected to short-term high temperature stress.