Effects of mechanical trauma of platelets and red blood cells on dynamic viscoelasticity of blood during clotting were examined. Two different methods were attempted to give the mechanical damage to blood cells. In one method, a flow apparatus consisting of a thin teflon tube and two sample reservoirs connecting to both ends of the tube was employed. After a blood sample repeated the shuttle flow many times through the tube, the dynamic rigidity modulus (G') and loss modulus (G") of the blood sample during clotting were measured. The mechanical trauma of platelets brought about the increase of G' and G" for clots of PRP and whole blood, in addition to shorter clotting time. Slight hemolysis of red blood cells (less than 0.5% of total hemolysis) only yielded shorter clotting time for whole blood. In other method, packed red blood cells were hemolyzed by exposing the ultrasonic wave (the amount of hemolysis was about 5% of total hemolysis). Addition of supernatant of hemolyzed red blood cells to PRP caused the platelet aggregation. Furthermore, a marked hemolysis brought about higher values of G' and G" for blood clot, in addition to shorter clotting time. These results suggest that the release of constituents from damaged red blood cells and alteration of cell surface due to mechanical damage would accelerate the coagulation sequence.