We investigated the effects of fibrin concentration on thrombus formation by using Monte Carlo simulations. Only coulomb forces were considered as the intermolecular potential of fibrin for simplicity. The motions of 1,000 fibrins were calculated in the simulations. The concentration was increased by decreasing the size of calculation domain. When the concentration was increased, the cluster size was suddenly increased over 0.03 [g/100ml]. In this study the threshold concentration of fibrin was discussed for thrombus formation.