We report a case of primary cardiac angiosarcoma in a 35 year-old woman. She presented with dyspnea, facial edema and neck vein distension during 1 month. The oval round large mass(8x4 cm) filling the right atrium infiltrated into the entire thickness of the lateral wall and extended to the vena cava and ventricle. The interatrial septum and pericardium seemed to be intact. Despite the surgical excision and adjuvant chemo-and radiotherapy, the patient died 8 months later due to tumor recurrence. The histologic findings of tumor varied from benign looking capillary proliferation mimicking granulation tissue to sarcoma composed of spindle cells. But anastomosing vascular channels lined by anaplastic polygonal cells and intracytoplasmic lumen containing red blood cells were characteristic findings. These tumor cells showed positive reaction to endothelial cell antigen and factor VIII-related antigen. The intercellular junctional complex and vascular channels formed by cytoplasmic process of tumor cells were identified by the ultrastructural study.