The ethyl acetate fraction of an aqueous alcoholic extract from the stem of Parthenocissus tricuspidata yielded 11 known compounds (1-11) and two new stilbene dimers parthenostilbenins A (12) and B (13) upon purification either by preparative TLC or reversed phase HPLC. The structures of the new isolates were identified using a combination of FAB-MS and NMR. These compounds were assessed for antioxidant activities in three different bioassay systems. Among them, piceatannol showed the strongest inhibitory activity in these assay systems. Two new compounds, parthenostilbenins A (12) and B (13) inhibited lipid peroxidation (IC (50) = 20.35 +/- 1.22 and 18.68 +/- 0.51 microg/mL, respectively) in a rat liver homogenate.