By using open top chamber fumigation method, this paper studied the effects of elevated O3 (approximately 80 nmol x mol(-1)) and CO2 (approximately 700 micromol x mol(-1)), single and in combination, on the active oxygen metabolism of Quercus mongolica leaves. Elevated O3 increased the superoxide anion radical (O2 divided by) generation rate, hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents, and electrolyte leakage (EL) (P < 0.05), but decreased the superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT) activities and ascorbate (AsA) content (P < 0.05); while elevated CO2 had no significant effects on the active oxygen metabolism of Q. mongolica leaves. The combination of elevated O3 and CO2 had less effects on the O2 divided by generation rate, H2O2 and MDA contents, and EL, indicating that elevated CO2 alleviated the oxidative stress of elevated O3 on Q. mongolica leaves, but increased the SOD, APX, and CAT activities and the AsA and total phenol contents, compared with elevated O3 (P < 0.05), indicating that elevated CO2 alleviated the negative effects of elevated O3 on Q. mongolica leaf antioxidant system.