Abstract: Abstract: Global atmospheric CO2 concentration has been increased from 280 μmol/mol to over 400 μmol/mol following the 19th century industrial revolution, and may even be over 800 μmol/mol at the end of this century, which may lead to drastic impacts on the structure and function of natural and managed ecosystems. We studied the effects of doubling CO2 concentration on physiological traits and water use efficiency of 5 crop species soybean (Glycine max (L.) Merr.), winter wheat (Triticum aestivum L.), kentucky bluegrass (Poa pratensis L.), perennial ryegrass (Lolium perenne L.), and tall fescue (Festuca arundinacea Schreb.) using 8 environmental growth chambers that could accurately control the CO2 concentration at 400 μmol/mol or 800 μmol/mol. Leaf gas exchange measurement was performed with a portable photosynthesis system (Li-6400XT). Photosynthesis vs intercellular CO2 curves were obtained to fit the maximum carboxylation rate of Rubisco, and the maximum electron transfer rate mediated ribulose bisphosphate regeneration. Meanwhile, we also obtained the stomatal conductance, intercellular CO2 concentration, transpiration rate, and leaf water use efficiency of crops. Results showed that doubling CO2 concentration significantly increased the net photosynthetic rate of perennial ryegrass (P<0.05), but barely affected that of the other 4 crops (all P>0.05). Our results from two-way ANOVA showed that crop species and CO2 concentration has significantly interactive effects on net photosynthetic rate, stomatal conductance, and transpiration rate of the 5 crops. Meanwhile, elevating CO2 concentration decreased the stomatal conductance and transpiration rate of crops except for soybean and winter wheat. As a result, elevating CO2 concentration increased the water use efficiency of perennial ryegrass, kentucky bluegrass, and tall fescue by 175%, 161%, and 74%, suggesting that grass species may be more sensitive to elevated CO2 concentration than soybean and winter wheat. Both the increased net photosynthetic rates and the decreased transpiration rate may contribute to the enhanced water use efficiency, whereas the contributions from the net photosynthetic rate and the transpiration rate depend on crop species. The increased leaf photosynthesis contributed the enhancement of water use efficiency for perennial ryegrass and tall fescue by 67% and 37%, respectively. Moreover, elevated CO2 concentration barely affected the relative growth rate, root/shoot ratio, and total biomass of soybean plants, but dramatically increased the root/shoot ratio, and total biomass of winter wheat. Meanwhile, we also found that elevated CO2 concentration had little effect on the relative growth rate, root/shoot ratio of the 3 grass species, while significantly enhanced the total biomass of tall fescue. These results suggested that crops in response to elevated CO2 concentration was obviously species dependent. Additionally, our results also showed positively linear relationships between leaf nitrogen content and transpiration rate of crops except for soybean. Overall, our results suggest that the responses of physiological traits and water use efficiency to doubling CO2 concentration depend on crop species under future climate change.