The partial pressure of CO2 in air decreases with the increase in altitude. Therefore, increase in molar concentration of CO2 is smaller at higher altitudes than at lower altitudes for increases in molar fraction of CO2. This study aimed to predict the effect of global CO2 increase on net photosynthetic rate of spring wheat (Triticum aestivum L.) at high altitudes. The net photosynthetic rate of spring wheat grown in Lhasa (3688 m above sea level), China, was compared with that of the same cultivar grown in Sapporo (15 m above sea level), Japan. At the current level of CO2, it was significantly lower in Lhasa than in Sapporo, and stomatal conductance, chlorophyll content (SPAD value) and apparent quantum yield were similar in both locations. The interaction of CO2 level and altitude was suggested; the amount of increase in net photosynthetic rate caused by increase in CO2 was smaller at high altitudes than at low altitudes. Lower CO2 partial pressure at higher altitude could explain the difference in net photosynthetic rate between altitudes, and the interaction of CO2 level and altitude.