We experienced thymolipoma which is a rare benign mediastinal tumor, an admixture of atrophic thymic tissue and matured adipose tissue. The patient was, 13 years old boy, admitted due to anterior mediastinal mass on routine chest X-ray and complained of mild exertional dyspnea. The physical examination revealed percussion dullness, decreased breathing sound at right anterior hemithorax. The chest PA and lateral views showed both paracardiac mass density and obliteration of retrosternal clear space. The chest CT Scan showed tubular structure containing of thymic tissue and enhanced fatty tissue, suggested thymolipoma. Operation was performed under the impression of thymolipoma. Through median sternotomy, the huge mass was resected. Grossly, the mass was yellowish fatty colored, measured 15X12X10cm, weighed 600gm. Light microscopic examination showed that thymic tissue containing of Hassal`s corpuscles were surrounded by matured adipose tissue, as the characteristic finding of thymolipoma. The postoperative course was uneventful.