Carcinoid tumor is very rare primary tumor in the thymus, first described by Rosai and Levine in 1972. Before Rosai and Levine, it was diagnosed as epithelial tumor of the thymus or bronchial adenoma-like tumor of the mediastinum. Recently the authors experienced a case of primary thymic carcinoid tumor diagnosed by histopathologic, histochemical, and electronmicroscopical study. A 54 years old policeman was admitted for surgical intervension, because of a chest mass that was detected in routine chest x-ray. He had been well being until the mass was found. On admission, physical examination was all within normal limit. Laboratory findings were within normal limit, except the fasting blood sugar was 141㎎/㎗, and postprandial sugar, 225㎎/㎗. An exploratory thoracotomy was performed, and an anterior mediastinal mass was easily removed. During the operation, any metastatic mass or other abnormalities were not observed. The mass was a well encapsulated multilobulated mass, measured 12.0×6.5×7.0㎝ and weighing 280gm. On cut section, it revealed homogeneous tan gray lobulated cut surface with firm consistency. Any necrotic and hemorrhagic areas were not found grossly. Microscopially this tumor consisted of diffuse pattern of monomorphic cells, or rosette-forming area, and ribbon-like arranged areas. Argyrophil stain revealed scattered argyrophil cells, and argentaffin stain was negative. Electronmicroscopic study revealed some membrane-bounded electron dense granule which were measured 50-150nm, in the cytoplasm. That were consistent with neurosecretory granules, found in the carcinoid tumor of the foregut.