Costus speciosus (C. speciosus) has several pharmacological properties including antioxidant activity. Recently we have shown that methanol extract of C. speciosus leaves of summer has maximum in vitro antioxidant activity. Aim of the present work was to isolate antioxidant compound from C. speciosus leaves. Summer sample of C. speciosus leaves were collected from the local market and identified by the taxonomist. Methanol extract of the leaves was prepared and processed for isolation of antioxidant compound. Acid hydrolysis, solvent treatment, chromatographic experiments followed by crystallization were done to isolate a compound. In vitro antioxidant activity of the isolated compound was measured by superoxide anion generation with the help of xanthine-xanthine oxidase assay, linoleic acid peroxidation assay as well as by DPPH photometric assay. Isolated compound showed significant in vitro antioxidant activity which was comparable to that of quercetin, a synthetic antioxidant. The isolated compound may, therefore, be used as natural antioxidant.