A two-dimensional particle method simulation of blood flow was carried out to investigate effects of stenosis on primary thrombogenesis due to platelet aggregation. In the employed particle method, the blood region was discritized by moving particles that have the characteristics of plasma and platelets. Two geometrically different models were constructed for straight and stenosed vessels. As a result, it was shown that platelets adhered to cover the whole area of the injured wall in the straight model. In the case of the stenosed model, platelets adhered on the downstream side of the injured wall and not on that of the upstream side. This was caused by the disturbance of the blood flow due to the geometrical nonuniformity of stenosis, leading to local growth of the thrombus. These results suggest that thrombogensis is influenced by the blood flow pattern depending on the vessel geometry.