The roots of two Paeoniaceae family members have long been used as traditional medicines in Korea, China, and Japan. Dry roots of Paeonia lactiflora and dry root bark of P. suffruticosa are used under the traditional names of Paeoniae Radix and Moutan Cortex, respectively. Both Paeoniae Radix and Moutan Cortex have been used as remedies for cardiovascular diseases, for improving blood circulation, or for other uses. It was postulated that both plants may contain common active constituents that contribute to inhibiting blood coagulation and/or platelet aggregation. Eighteen compounds, which have been reported to be present in both plant medicines, were evaluated for their effects on platelet aggregation and blood coagulation. Paeonol (5), paeoniflorin (9), benzoylpaeoniflorin (11), and benzoyloxypaeoniflorin (12) were found to be the major common active constituents and they would collectively contribute to improving blood circulation through their inhibitory effects on both platelet aggregation and blood coagulation. In addition, methylgallate (4), (+)-catechin (7), paeoniflorigenone (8), galloylpaeoniflorin (13), and daucosterol (16) may also take part in improving blood circulation by inhibiting ether platelet aggregation and/or blood coagulation.