Costus speciosus (C. speciosus) has long been medicinally used in different systems of medicine. The plant has several pharmacological properties including antioxidant activity. Aim of the present work was to see the effects of extraction solvents on in vitro antioxidant activity of C. speciosus leaves. Leaves of C. speciosus were collected from the local market and identified by the taxonomist. Extracts of the leaves were prepared separately using acetone, methanol, ethanol, chloroform, ethyl acetate and petroleum ether. With the help of xanthine-xanthine oxidase assay, linoleic acid peroxidation assay and DPPH photometric assay, in vitro antioxidant activities of all extracts were checked. Total phenol, ascorbic acid, flavonoids and carotenoids contents of the extracts were also determined. Results showed that methanol extract of C. speciosus leaves had maximum in vitro antioxidant activity in comparison to other solvent extracts. This was due to presence of high amount of total phenol in methanol extract. Methanol extract of C. speciosus leaves, therefore, may be further investigated in search for natural antioxidant compounds.