Eupatorium adenophorum (E. adenophorum) was hydro-distilled and the obtained essential oil was analyzed by gas chromatographyâmass spectrometry (GCâMS). Twenty six essential oil constituents comprising sesquiterpenes and monoterpenes were identified. Î³-Cadinene was most abundant followed by germacrene-D and Î³-elemene. E. adenophorum leaf powder was extracted with different solvents. Three pure constituents were isolated and identified by chromatographic and spectroscopic techniques. The antioxidant activities of the oil and cadinenes were evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and the ferric reducing ability assay (FRAP), together with three antioxidant standards, i.e. ascorbic acid, tert-butyl-4-hydroxy toluene (BHT), and gallic acid. The antioxidant activity of essential oil was comparable with the standards. The essential oil and the cadinene sesquiterpene-rich extract of E. adenophorum have exhibit potential antioxidant activities. Wide availability of the plant and the potential antioxidant activity makes it a potential natural antioxidant.