ABSTRACT: from this article we see that why COVID-19 causes thrombosis, A thrombus is a mixture of platelets, erythrocytes and fibrin protein, which begin to accumulate on the wall of the damaged vessel. gradually it thickens and hardens, but in general it resembles a viscous substance, similar to jelly,it can be unambiguously stated that with this virus, from the very beginning, there is an activation of hemostasis, intravascular blood coagulation and thrombus formation in small-caliber vessels of vital organs. In this case, not only the lungs are damaged, but the blockade of microcirculation and its irreversible nature determine the outcome of the disease.