Tumor thrombus into the superior vena cava have been rarely reported in cases with mediastinal tumors. These tumors are frequently invasive and continuous from the main tumor that normally shows direct vessel wall invasion, but not in this case. In some cases, the tumor thrombus can be removed through a simple venotomy. To prevent superior vena cava and the left innominate vein stenosis, we used a pericardial patch to close the venotomy site. At this case we report a thymic carcinoma with superior vena cava syndrome, which was caused by a tumor thrombus in the superior vena cava without vessel wall invasion, an even more rare condition.