A 47-year-old female was admitted for an evaluation of a left mediastinal mass. The chest X-ray performed 16 months ago was normal, but the chest X-ray upon admission showed a large mass adjacent to the anterior aspect of the left hemidiaphragm. The CT scan demonstrated a large mass with a fat density in the left lower hemithorax. A focal diaphragmatic defect behind the xiphoid process was suspected. A thoracoscopic examination revealed omental herniation through the diaphragmatic defect. Therefore, a left thoracotomy was performed and the defect was repaired. We believe that a differential diagnosis should be needed to include a diaphragmatic omental hernia when a fat density mass is observed in the mediastinum.