Abstract: Abstract: To further understand the response mechanism of stomatal traits and leaf gas exchange of maize (Zea mays L.) to elevated CO2 concentration and high temperature stress, we examined the combined effects of double atmospheric CO2 concentration and high temperature on plant growth, stomatal traits, and leaf gas exchange parameters of maize grown at six environmental growth chambers with three temperature regimes ((day/night) 25/19 ℃, 31/25 ℃, and 37/31 ℃) and two CO2 concentrations (400 μmol/mol (C400) and 800 μmol/mol (C800)), respectively. These environmental growth chambers were controlled with the same environmental factors, where the Photosynthetic Photon Flux Density (PPFD) was 1 000 μmol/(m2·s) and the relative humidity was 50% to 60%. In each chamber, we measured the net photosynthetic rate (Pn), transpiration rate (Tr) and stomatal conductance (gs) using a portable photosynthesis system (LI-6400XT, LI-COR Inc., Lincoln, NE, USA). The maximum photochemical efficiency of PSII (Fv/Fm) was estimated by measuring chlorophyll fluorescence with a photosynthesis efficiency analyzer (Handy PEA, Hansatech Instrument Ltd., Norfolk, UK). In addition, we also measured the leaf area, leaf length, and leaf width as well as the final leaf number of maize plants. The results showed that: 1) The stomatal density of maize was significantly increased by temperature (P < 0.001), but barely affected by CO2 concentration (P > 0.05). Meanwhile, the increase of stomatal density on the adaxial leaf surface was significantly higher than that on the abaxial surface of maize leaves, which indicated that the response of stomatal density on the adaxial leaf surface to elevated temperature might be more sensitive than that on the abaxial leaf surface of maize. Furthermore, the results also showed that the increase of stomatal density was accelerated with the elevated temperature on both the adaxial and abaxial leaf surfaces of maize. 2) The leaf transpiration rate were significantly enhanced by 57% and 84% with increasing growth temperature(day/night) from 25/19 ℃ to 37/31 ℃ at both the ambient (C400) and double atmospheric CO2 concentrations (C800). And there were linear positive correlation between the stomatal density on the adaxial and abaxial leaf surfaces and the transpiration rate of maize plants (adaxial surface, R2=0.69; abaxial surface, R2=0.71), which indicated that the leaf transpiration rate could be improved by adjusting stomatal density to optimize leaf gas exchange efficiency under high temperature environment. 3) The net photosynthetic rate (Pn) of maize was also significantly enhanced by 23% and 21% with increasing temperature(day/night) from 25/19 ℃ to 31/25 ℃, however, the Pn drastically declined by 24% and 13% when the temperature increased from 31/25 ℃ to 37/31 ℃ at both CO2 concentrations, which indicated that the high temperature (37/31 ℃) might result in physiological damages to the sites of photosynthetic reaction center, but this thermal stress from high temperature could be alleviated by elevated CO2 concentration. Also, the maximum photochemical efficiency (Fv/Fm) of maize drastically decreased at both CO2 concentrations when the temperature was elevated from 31/25 ℃ to 37/31 ℃. Overall, The results in this study maybe of significance for further understanding the potential mechanisms and processes of elevated atmospheric CO2 concentration mitigating the physiological damage of high temperature to maize plants under future climate change.