To study the impact of elevated atmospheric CO₂ concentrations on activities of anti-oxidative enzymes and lipid peroxidation of trees, the superoxide anion (O2^-•) generating rate, malondialdehyde (MDA) content, activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR), and ascorbic acid (ASA) content were periodically analyzed in leaves of Ginkgo biloba exposed in open-top chambers to either ambient (≈350μmol mol^-1) or elevated (700μmol mol^-1) CO₂ concentrations in urban area for a growing season. The results show that elevated CO₂ exposure in the short-term reduced generating rate of superoxide anion radical and content of hydrogen peroxide. Malondialdehyde (MDA) content as an index of lipid peroxidation was also decreased. The activities of SOD, APX and GR, and ascorbate content were increased by high CO₂ exposure. However the results were reversed by the long-term elevated CO₂ exposure. Generating rate of superoxide anion radical and content of hydrogen peroxide slight increased, the activities of SOD, APX and GR tiny declined and ASA content increased. These results were not significant difference compared to control. It is concluded that the activities of antioxidant system in Ginkgo biloba increased and the ability of scavenging reactive oxygen enhanced. However, the antioxidant ability might be declined by the long-term high CO₂ exposure.