Mediastinal teratoma is one of the most common lesions found in the anterior mediastinum, accounting for $8sim13%$ of all mediastinal tumors. This tumor is incidentally detected by routine chest roentgengography, but pericardial perforation or pleural effusion occurs rarely. In our patient cardiac tamponade was developed due to anterior chest wall contusion, we confirmed the anterior mediastinal tumor. Vital signs were stabilized after the pericardiocentesis, and the patient underwent the tumor resection in the anterior mediastinum for a definite treatment. On histologic examination, the tumor revealed cystic structures composed of mature squamous epithelium, pilosebaceous glands, mature fat tissue, gastrointestinal mucosa, respiratory epithelium, and pancreatic tissues.