Oxidative stress may precede and accelerate the development of diabetes and then diabetic complications. Antioxidant action may be an important property of plant medicines associated with diabetes. So we analyzed antioxidant activity of the aqueous extracts of leaves of six medicinal plants namely Gymnema sylvestre (Asclepiadaceae), Coccinia indica (Cucurbitaceae), Catharanthus roseus (white), C. roseus (purple) (Apocynaceae), Momordica charantia var. charantia and M. charantia var. muricata (Cucurbitaceae) which are used to treat diabetes mellitus in India. The antioxidant activity was assessed in different systems of assay e.g. DPPH radical scavenging assay, superoxide scavenging assay, hydroxyl radical scavenging assay and assay to determine prevention of lipid peroxidation by plant extracts. The extracts were found to have different levels of antioxidant activity in the systems tested. Highest DPPH radical scavenging activity was found in C. roseus (white variety) IC50 value being 241.13 μg/ml. C. roseus (purple variety) (IC50 value 104.04 μg/ml) had highest superoxide radical scavenging activity. Highest hydroxyl radical scavenging activity was found in C. indica (IC50 value 552.93 μg/ml). IC50 value of G. sylvestre which prevented lipid peroxidation the most is 1390.07 μg/ml. Total antioxidant capacity equivalent to ascorbic acid and gallic acid was also determined. Total phenol content equivalent to gallic acid and total flavonoid content equivalent to catechin were determined. There was no correlation between total phenolics or flavonoids contents and antioxidant activity of the plants analysed. Although the antidiabetic property of G. sylvestre is due to gymnemic acid, present study reveals that the antioxidant property is due to the presence of phenolic compounds.