The gradual increases in the atmospheric CO_2 concentration and temperature are the two primary characteristics of global climate change.However,the rising atmospheric CO_2 concentration could alleviate the negative effects of heat stress.Therefore,the aim of this study was to explore the mitigating effects of elevated CO_2 on the growth and antioxidant system a perennial grass under heat stress.Tall fescue(Festuca arundinacea cv.Barlexas')was exposed to either ambient CO_2 concentration(400mumol/mol)or elevated CO_2 concentration(800mumol/mol)under optimal growth temperature(25/15℃ day/night)or elevated temperature(35/ 25℃ day/night)conditions.After 28 days of the experimental treatment,heat stress caused a significant reduction in relative growth rate(RGR),leaf net photosynthetic rate(P_n),number of green leaves,antioxidant enzyme activities[(superoxide dismutase,(SOD),catalase(CAT),peroxidase(POD),and ascorbate peroxidase(APX)],as well as an increase in electrolyte leakage(EL),and the contents of malondialdehyde(MDA) and reactive oxygen species(ROS)(H_2O_2 and O_2~-·).Elevated CO_2 mitigated these negative effects of heat stress in tall fescue.Compared with plants grown under heat stress and ambient CO_2,those grown under heat stress and elevated CO_2 had lower leaf EL and MDA contents(72% and 39%lower,respectively);the P_n,F_v/F_m,and number of green leaves were increased by 1.74-fold,17%,and 1.65-fold,respectively;and the ROS contents were significantly lower(H_2O_2 and O_2~-·contents decreased by 46% and 31%,respectively).These results demonstrated that elevated CO_2 can improve the heat tolerance of tall fescue through increased photosynthetic capacity,cellular membrane stability,and decreased ROS accumulation.