Abstract: In course of investigation on natural antioxidants, the present study was aimed to report the antioxidant activities, both in vitro and in vivo, of the crude methanolic extracts of the whole plant of Oxalis corniculata Linn along with its various organic fractions. The different assay methods, including total antioxidant activity, scavenging free radical, authentic peroxynitrite, nitric oxide and reducing power assessment were used to evaluate the antioxidant potential of the crude extract and its organic fractions. The ethylacetate (EtOAc) fraction, showed strong activity in all the model systems tested and in peroxynitrite model this fraction (IC50 value of 2.29±0.18 μg mL-1) exerted three-fold stronger activity than standard penicillamine (IC50 value of 6.20±0.32 μg mL-1). The reducing power of the extract was found to be concentration dependent. The administration of the extract/fractions at a dose of 250 and 500 mg kg-1 body weight to the male Wistar rats increased the percentage inhibition of reduced glutathione, superoxide dismutase and catalase significantly. Whereas, lipid peroxidation level in hepatotoxic rats markedly decreased at a dose of 500 mg kg-1 body weight after 7 days. The total phenol and flavonoid content were also measured in the crude extract along with its organic fractions. The Brine shrimp lethality bioassay was used to determine the toxicity of the extracts and Vincristin sulphate was used as positive control. The dichloromethane (CH2Cl2) fraction showed highest activity (LC50 value of 29.02±1.16 μg mL-1) and other showed activity in the order of: EtOAc fraction >n-BuOH fraction> MeOH extract > aqueous fraction. Taken together, these results suggest that O. corniculata extract has strong antioxidant properties and further validate the traditional use of this plant.