Reactive oxygen species (ROS) are involved in a wide spectrum of diseases including chronic inflammation and cancer. In the present study, the 70 % methanolic extract of Gmelina arborea (GA) stem bark was tested for its antioxidant activities in both in vitro and in vivo models. The GA extract was shown to possess antioxidant activities as it scavenged superoxide radical and inhibited lipid peroxidation. The extract also showed reducing property as evidenced by ferric reducing power assay. The sodium fluoride (600 ppm/day for 14 days) induced oxidative stress model was used for evaluating the in vivo antioxidant potential of the plant extract. The sodium fluoride intoxication resulted in the impairment of antioxidant defense system of blood and liver as evidenced by the reduced levels of superoxide dismutase, glutathione peroxidase and glutathione. The lipid peroxidation in the liver tissue was elevated indicating the free radical mediated membrane damage. The treatment with plant extract could reduce lipid peroxidation and enhance antioxidant defense system in a significant manner.