Seven flavonoids were isolated from the butanol fraction of the methanolic extract of the aerial parts of Cynanchum acutum L. (Asclepiadaceae). All of which have been isolated for the first time from the genus Cynanchum. Their structures were established as quercetin 3-O-beta-galacturonopyranoside (1), quercetin 7-O-beta-glucopyranoside (2), tamarixtin 3-O-beta-galacturonopyranoside (3), kaempferol 3-O-beta-galacturonopyranoside (4), 8-hydroxyquercetin 3-O-beta-galactopyranoside (5), tamarixtin 3-O-alpha-rhamnopyranoside (6), and tamarixtin 7-O-alpha-arabinopyranoside (7) on the basis of their chromatographic properties, chemical and spectroscopic data. The major isolated flavonoids 1, 2 and 3 were found to exhibit significant antioxidant and antidiabetic activities (by measuring blood glucose and insulin levels). This is the first report about the antioxidant and antidiabetic activities of compounds 1-3.