Intracardiac hemangiopericytomas are rare tumors which originates from the pericyte in the external wall of capillaries. 1) The tumors are known to usually develop in the lower extremities, pelvic cavity and retroperitoneum, 2) but are very rare in the heart. 3) The symptoms and signs of a hemagiopericytoma depend on the size and location of the tumor. 2) A hemagiopericytoma has a high potential for local recurrence and metastasis, so regular follow-up is needed following surgical excision. 2) A 36-year-old man presented with shortness of breath and chest discomfort. Before operating, a chest CT scan showed that a compressive collapse of the left lung had developed next to a large mediastinal tumor. Because of impending respiratory failure due to collapse of the left lung, an operation was performed. The operation showed that the mediastinal tumor was a large loculated hemopericardium accompanied by pericardial bleeding. A hematoma evacuation with a pericardiectomy was performed, and the pathology of the thickened pericardial wall revealed a malignant hemangiopericytoma. The patient has followed up for 6 months without symptoms or sign of tumor recurrence following the radiation therapy.