There have been a number of investigations on coagulation reactions of blood as well as on coagulation factors including fibrinogen and thrombin. With the progress of clotting, the viscosity and rigidity of blood increase, facts related to the clot structure of fibrin. Therefore, rheological measurements make it possible to investigate the process of blood clotting as well as the properties of the fibrin clot. In this paper, our rheological studies on blood coagulation are summarized. The network structure of fibrin clots formed under different conditions is discussed from the kinetic analysis of the change of dynamic rigidity modulus during clotting. Also it will be shown that rheological techniques make it possible to analyze the initial coagulation reaction of blood in contact with components constituting the vascular vessel. The analysis indicates that a coagulation factor in plasma is activated on the erythrocyte surface.