The mediastinal lipoma is an extremely rare tumor in children. Since the orininal description by Fothergill in 1781, about 120 cases have been reported in the world literature. Of these, less than 10 have been reported in children below the age of 12. We had experienced a posterior mediastinal lipoma in 3 year old boy. He did not complain of any subjective symptoms. Routine X-ray film revealed a huge round homogenous mass density in the posterior mediastinum. On posterior thoracotomy incision,bright yellow,well encapsulated, partly 1obular adipose mass was found in the posterior mediastinum. The tumor mass was removed easily,being proved to be lipoma on histopathologic examination. His postoperative course was uneventful and discharged on the 12th postoperative day.