To determine the photosynthetic characteristics of C3 plants and their sensitivity to CO2 at different altitudes on the Tibetan Plateau, hulless barley (Hordeum vulgare L. ssp. vulgare) was grown at altitudes of 4,333 m and 3,688 m. Using gas-exchange measurements, photosynthetic parameters were simulated, including the maximum net photosynthesis (Pmax) and the apparent quantum efficiency (α). Plants growing at higher altitude had higher net photosynthetic rates (PN), photosynthesis parameters (Pmax and α) and sensitivities to CO2 enhancement than plants growing at lower altitude on the Tibetan Plateau. The enhancements of PN, Pmax, and α for plants growing at higher altitude, corresponding with 10 μmol(CO2) mol-1 increments, were approximately 0.20∼0.45%, 0.05∼0.20% and 0.12∼0.36% greater, respectively, than for plants growing at lower altitude, respectively, where CO2 levels rose from 10 to 170 μmol(CO2) mol-1. Therefore, on the Tibetan Plateau, the changes in the photosynthetic capacities and the photosynthetic sensitivities to CO2 observed in the C3 plants grown above 3,688 m are likely to increase with altitude despite the decreasing CO2 partial pressure. and Y. Z. Fan ... [et al.].