The dayflower, Commelina communis L., contains 1-deoxynojirimycin (DNJ) and (2R,3R,4R,5R)2,5-bis(hydroxymethyl)-3,4-dihydroxypyrrolidine (DMDP), potent alpha-glucosidase inhibitors. The extracts and powder of this herb are important food materials for prophylaxis against type 2 diabetes. Eleven flavonoid glycosides as antioxidants, isoquercitrin, isorhamnetin-3-O-rutinoside, isorhamnetin-3-O-beta-D-glucoside, glucoluteolin, chrysoriol-7-O-beta-D-glucoside, orientin, vitexin, isoorientin, isovitexin, swertisin, and flavocommelin, were identified from the aerial parts of C. communis. Their antioxidant activities were measured using in vitro assays employing the 1,1-diphenyl-2-picrylhydrazyl radical- and superoxide radical-scavenging assays. The results showed that glucoluteolin, orientin, isoorientin, and isoquercitrin are the predominant antioxidants in this herb. Moreover, isoquercitrin, isorhamnetine-3-O-rutinoside, vitexin, and swertisin inhibited the activity of alpha-glucosidase from rat intestine.