In the courses of in vitro screening for the antioxidant effect of the various extracts from medicinal plants, n-BuOH soluble extract of the seeds of Xanthium strumarium was found to exhibit distinctive antioxidant activity. Further purifications of this extract as guided by in vitro antioxidant assay afforded caffeic acid and 3,5-di-O-caffeoyl quinic acid, which scavenged 1, 1-diphenyl-2-picrylhydrazyl (DPPH) radical in a dose-dependent manner of which IC50 values were 23.4 μg/ml (132.9 μM) and 29.8 μg/ml (57.9 μM), respectively.