This study attempted modeling of fibrin network formation process in red thrombus with a two-dimensional particle method simulation of blood flow between parallel plates. Blood flow simulations were carried out based on the two different models of fibrin formation. The first model assumed fibrin formation according to the elapsed time with respect to platelet aggregation. A simulation result on this model demonstrated that fibrin began to form near the injured blood vessel wall where the platelet aggregation was assumed to occur, and that this led to formation of firm thrombus by platelets and fibrin. The second model assumed fibrin formation according to the blood flow. In this case, fibrin began to form near the surface of already aggregated platelets. Because stiffer fibrin formed on softer platelet thrombus, the platelet thrombus that formed at an initial stage of simulation collapsed. After the collapse fibrin adhered to the injured vessel wall, leading to formation of firm thrombus as well as in the first model.