A primary pericardial tumor is very rare. A 77-year-old woman was admitted to our hospital with chief complaint of exertional dyspnea due to large amount of pericardial effusion. She was finally diagnosed as pericardial undifferentiated carcinoma without definite histopathologial, immunochemistry feature. Despite palliative radiation therapy, the patient died of multiple organ failure. The prognosis of primary pericardial undifferentiated carcinoma is known to be very poor, especially in old people.