A reaction order for the formation of fibrin network in the fibrinogen–thrombin solution is determined by an analysis of clotting curves obtained by measurements of dynamic viscoelasticity at the various concentrations of fibrinogen and thrombin. It is concluded from a rheological point of view that clotting curves of elastic modulus and loss modulus can be represented by superposition of two first-order reaction processes with different rate constants respectively. The same conclusion is obtained for human and bovine plasma with different rate constants. 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 concentrations of fibrinogen and thrombin. The amplitude dependence of the elastic modulus of the gel also supports the presence of the two processes. The elastic modulus in the first process is proportional to the number of cross links between fibrin fibers, and that in the second process is related to the non-linear elongation of fibrin fibers between crosslinks. The loss modulus in the first process is associated with the polymerization of fibrin, that in the second process is related to the increasing interaction between crosslinked fibrin fibers.