Thromboembolism is a major cause of morbidity and death following implantation of cardiac prosthetic devices. Effective systemic anticoagulation is very important. The presence of thrombus can often be detected by pulmonary edema associated with the disappearance of valve clicks. 2-D echocardiography and phonocardiography are also valuable tools. The most common treatment is reoperation and replacement after discovery early. We have experienced one death of fatal thromboembolism after St. Jude valve replacement was done in 48 years old male diagnosed of severe mitral stenosis. He was treated with warfarin, aspirin, ticlopidine for 10 weeks but died suddenly. At autopsy, valve dysfunction was seen due to organic thrombus arising from atrial septum and confirmed with microscopic findings.