In order to reveal effect mechanism of elevated carbon dioxide（CO2 ）and ozone（O3）concentration singly and in combination on plants reactive oxygen species（ROS）metabolism and anti-oxidative enzymes activities, open top chambers (OTCs) were utilized to investigate the change of ROS production rate, content, lipid peroxidation extent, anti-oxidative enzymes activities net photosynthetic rate in maize（Zea mays L.）leaves and grains yield under elevated CO2 and O3 concentration singly and in combination treatment. The results indicated that elevated CO2 concentration（(550±20)μmol?mol-1）induced a decrease on superoxide anion（O-?2） production rate, hydrogen peroxide（H2O2）content, content of malondialdehyde（MDA）, relative electrical conductivity and a increase on net photosynthetic rate and the activities of superoxide dismutase（SOD）, catalase （CAT）, peroxidase（POD）in maize leaves and also a elevation on one-hundred-grain weight, grains number of per spike during the whole growth stage. While elevated O3 concentration exposure（80±10nmol?mol-1）showed the opposite effects, which resulted in higher O-?2 production rate, H2O2 content, content of MDA, relative electrical conductivity and lower net photosynthetic rate and activities of SOD, CAT, POD and a reduction on one-hundred-grain weight, spike grains number of maize compared with the control. But under CO2 and O3 concentration combined（(550±20)μmol?mol-1+(80±10)nmol?mol-1）treatment, compared with the control, the net photosynthetic rate, H2O2 content, SOD activity were higher earlier and lower later during the growth stages, content of MDA, relative electrical conductivity, CAT activity were enhanced and POD activity was weakened, O-?2 production rate was almost unchanged in the leaves of maize, while one-hundred-grain weight and grains number of per spike in maize increased unsignificantly. The results show that ROS metabolism rates are restrained and anti-oxidative enzymes activities, photosynthesis, grains yield are enhanced by elevated CO2 concentration, which presents protective efficacy on maize, while elevated O3 concentration accelerats ROS metabolism rates, reduces anti-oxidative enzymes activities, photosynthesis and grains yield, which represents injured effects to maize. In combination of elevated CO2 and O3 concentration, the beneficial effects of CO2 on maize are more than compensating the negative effects imposed from O3, and the latter in its turn partly counteracted the positive effects to maize of the former.