The reaction order in the formation of fibrin network in the fibrinogen-thrombin solution is determined by the analysis of clotting curves obtained by dynamic viscoelasticity measurements at various concentrations of fibrinogen. It is concluded from the rheological point of view that the clotting curves for elastic modulus and loss modulus can be represented by superposition of two first-order reaction processes with different rate constants respectively. The rate constants obtained from the loss modulus are larger than those from the elastic modulus. The saturated values of the elastic moduli in the two processes increase with the increase of the concentration of fibrinogen. The amplitude dependence of the elastic modulus of the gel supports the presence of the two processes.The elastic modulus in the first process is proportional to the number of crosslinks between the fibrin fibers, and that in the second process is related to the non-linear elongation of fibrin fibers between the crosslinks. The loss modulus in the first process is associated with the polymerization of fibrin, and that in the second process is related to the increasing interaction between the crosslinked fibrin fibers.