The Blood coagulation system was converted to a mathematical model which was described using differential equations. The calculated output patterns of the mathematical model against various input stimulations were compared with results of in vitro assay. The simulated results of (1) Hemophilia A, (2) anticoagulation effect of antithrombin III and (3) anticoagulation effect of heparin corresponded to the results of in vitro assay and clinical reports. However, the simulated result of (4) anticoagulation effect of synthesized arginine derivative No. 805 (MD-805) did not correspond to the results of in vitro assay. Therefore, a new series simulation of MD-805 was done, supposing that MD-805 had an inhibitory activity not only on coagulation factor IIa but also VIIa. The new simulation pattern closely resembled the results of in vitro assay. From these facts, it was theoretically indicated that MD-805 also has an inhibitory activity on VIIa.