Mediastinal teratoma is a tumor that thoracic surgeons made an operation much less commonly than other mediastinal masses and most of them are asymptomatic. But very rarely, this tumor invades the pleura and pericardium resulting in pleural effusion, pericardial effusion and cardiac tamponade in severe cases. The mechanism of invasion and perforation of the tumor is unknown and tumor-consisting tissue factor is suspected of a cause. In this case, we operated on a patient whose anterior mediastinal teratoma invaded and perforated pericardium and pleura resulting in pericardial effusion and pleural effusion. The patient was improved and discharged with no problem after resection of mass and involved pericardium.