We report a case of anterior mediastinal lipoma. A 71-year-old female was admitted for cough. A fatdensity tumor from anterior mediastinum to left thoracic cavity was found by chest X-ray, chestcomputed tomography （CT） and magnetic resonance imaging （MRI）. Defect of left dorsal diaphragmwas suspected and there was a possibility that the tumor was connected to retroperitoneum.Under the preoperative diagnosis of a benign huge mediastinal lipoma, we conducted an operation. At1st, we observed by thoracoscopy and made sure that the mass was primary anterior mediastinal tumorand not connected to retroperitoneum. Through the median sternotomy, we completely resected thetumor with thymus. The tumor showed 27 cm in diameter, and histopathological diagnosis of the tumorwas benign lipoma.Lipoma of the mediastinum is very rare and about 0.3％ of all mediastinal tumors. It is sometimesdifficult to distinguish huge lipoma from liposarcoma only by clinical examinations such as CT scan orMRI. We evaluated the condition of the tumor by thoracoscopic observation, and the tumor was safelyand completely resected by median sternotomy.