Effect of season on in vitro antioxidant activity of Costus speciosus (C. speciosus) leaves was studied. Leaves of C. speciosus of different seasons were collected from the local market and authenticated by the taxonomist. In vitro antioxidant activity of the leaves was measured by superoxide anion generation with the help of xanthine-xanthine oxidase assay, linoleic acid peroxidation assay and DPPH photometric assay. Amount of total phenols present in the leaves of different seasons was also estimated. Results showed that in vitro antioxidant activity of the leaves of C. speciosus was maximum during summer (March – May) in comparison to other seasons of the year. Amount of total phenols present in the leaves was also found maximum in summer. In vitro antioxidant activity of C. speciosus leaves, therefore, was due high amount of total phenols in the leaves. Leaves of C. speciosus of summer may be further investigated to get natural antioxidant compound.