A 66-year-old male was referred to our department with mediastinal tumor. The chest X-ray and the chest CT showed a large tumor adjacent to pulmonary artery and descending aorta. Cavogram revealed a filling defect of the SVC. He underwent complete resection of the tumor, resection of the SVC and reconstruction of the SVC with ringed PTFE. The main tumor was 12 x 7 cm in size and the tumor in the SVC was 20 x 15 mm in size. Histopathological findings showed a lymphocytic thymoma. Compact invaded nest of tumor cell was found in the innominate vein. The tumor in the SVC was invasive to the intima of the SVC, but its media and adventure were intact. This case is rare among patients with invasive thymoma as far as the growth is concerned.