A 58-year-old female was admitted with an abrupt onset of chest and back pain. The CT scan of the chest showed aortic dissection of the ascending aorta and proximal aortic arch, but the false lumen of the aortic dissection had already been occluded by a blood clot. After admission, she complained of chest pain with hemoptysis and presented facial edema and the distention of the neck veins. The pulmonary angiogram showed complete occlusion of the right pulmonary artery at the proximal segment. These findings were interpreted as pulmonary embolism. She was treated with intravenous heparin and urokinase, but these treatments did not demonstrate any improvement. She underwent a surgical exploration on the fourth hospital day. During surgery, the right pulmonary artery was discovered to be compressed and occluded by the large dissecting aneurysm of the ascending aorta. In addition, hematoma was seen between the right pulmonary artery. The ascending aorta and pulmonary trunk, which was injured in the operative procedure, were replaced with an artificial graft successfully. Postoperative pulmonary angiogram showed no stenosis of right pulmonary artery. The occlusion of the pulmonary artery by an acute dissecting aneurysm is an extremely rare complication and it is often wrongly diagnosed as pulmonary embolism. In such cases, the correct diagnosis and prompt surgical treatment is essential and antithrombolytic and anticoagulant therapy should be avoided.