Many mediastinal masses seen radiologically are clinically asymptomatic. Only 55 to 65 percent of such lesions are asymptomatic, and of these many have only nonspecific symptoms by rupture, infection and pressure of surrounding organs. This report presents one case of a mediastinal teratoma which complicated an empyema. The patient was 7-year-old female and chief complaints were fever, left chest pain and general ache. In her past history, lung decortication and drainage procedure under the diagnosis of an empyema were carried out, 5 years ago, but she had not been improved, And so, explothoracotomy was done. At the time of operation, a mass of adult fist size was placed in the left anterior mediastinum and covered with the mediastinal pleura. Extirpation of the tumor and decortication were carried out and the left lung was remained.