A 59-year-old man was referred to our department for evaluation of an asymptomatic abnormal mediastinal shadow detected on chest roentgenogram. Computed tomography (CT) of the chest showed an anterior mediastinal tumor with ring calcification. Thymectomy through a median sternotomy was performed. Pathologic examination showed an encapsulated type B2 thymoma with ring calcification. Microscopically, the calcified layer was within the fibrous capsule layer. A part of the fibrous capsule was thought to become calcified like a ring. Although focal calcification is sometimes found in thymomas, ring calcification is rare.