Background: Platelets, coagulation cascade including thrombin and fibrin, and fibrinolysis by plasmin along with blood flow are known to play regulatory roles in thrombogenesis. However, quantitative and interactive contribution of these factors are not fully understood. Method: We developed a computer simulation model of thrombi at sites of vessel injury by implementing quantitative parameters of blood flow, platelet, coagulation cascade and fibrinolysis. In this model, we defined thrombi as the area where the amount of activated platelet become 80% or more of the global platelet count of 3.0х10 5 /mm 3 upon vessel damage We have measured the 3-dimensional size of thrombi as defined in various conditions with various parameters of flow velocity, rate of platelet activation by thrombin, rate of thrombin production on the surface of activated platelets, rate of fibrinolysis and rate of plasmin production. Results: Rate of platelet activation by thrombin had the largest influence on the size of thrombi under low flow rate condition of 0.5 (cm/sec) (Fig1 A). When blood flow rate increased to 2.0 (cm/sec), the absolute importance of this parameter decreased (Fig1 B). Conclusion: Our results show that the rate of platelet activation by thrombin, which are signaled with PAR-1 receptor in human, has a marked effect on the size of thrombi in low flow rate condition suggesting the benefit of blocking this receptor in low blood flow condition such as venous thrombosis.