Many cases with congenital heart disease are presented by intracardiac thrombosis. Most common site of is the right atrium. Insertion of a venous catheter in the inferior vena cava is one of the risk factor for thrombus in the right atrium. There are many causes that affect the occurrence of intracardiac thrombosis. There are genetic, mechanical or infectious causes. The association between the abnormal development of blood vessel and the thrombus formation was studied. In this article, the prothrombin G20210A mutation is responsible for thrombus.