A 19-year-old man was admitted to our hospital with anterior mediastinal cystic tumor. Subtotal thymectomy with the tumor was performed. The tumor was localized inside the thymus without outer infiltration. It weighed 145 g and measured 9 x 8 x 5 cm. The cut surface was cystic and spongy, partially with a solid focus. Pathological findings of the solid area was compatible with mucoepidermoid carcinoma of the thymus. The histogenesis of mucoepidermoid carcinoma of the thymus was discussed.