In order to clarify the anti-thrombosis activity of rhubarb, we investigated the effect of stilbene derivatives from rhizomes of Rheum undulatum on cyclooxygenase activity. Stilbene derivatives (desoxyrhaontigenin, rhapontigenin, piceatannol) exhibited the inhibitory effects on COX-1, and desoxyrhaponigenin showed inhibitory effect on COX-2. These inhibitory effect may partially contributed to anti-thrombosis activity of rhubarb.