Free radical stress leads to tissue injury and progression of disease conditions such as arthritis, hemorrhagic shock, atherosclerosis, diabetes, hepatic injury, aging and ischemia, reperfusion injury of many tissues, gastritis, tumor promotion, neurodegenerative diseases and carcinogenesis. Safer anti-oxidants suitable for long term use are needed to prevent or stop the progression of free radical mediated disorders. Herbal medicine provides a foundation for various traditional medicine systems worldwide. The Sida species is one of the most important families of medicinal plants in India. Hence, the present study was aimed to investigate the possible anti-oxidant potential of Sida rhombifolia extracts for 30 days on adjuvant induced arthritis in experimental rats. The altered levels of hematological parameters were reverted to near normal levels, especially the elevated rate of erythrocyte sedimentation was significantly reduced by S. rhombifolia extracts in experimental rats. Oral administration of root and stem of S. rhombifolia extracts significantly increased the levels of thiobarbituric acid reactive substances and activities of catalase and glutathione peroxidase and decreased the levels of reduced glutathione and superoxide dismutase activity in arthritis induced rats. The free radical scavenging activity of the plant was further evidenced by histological and transmission electron microscopy observations made on the hind limb tissue.