To prevent graft injury at the thoracic surgery after coronary artery bypass grafting (CABG), thymus is often used to cover the grafts. We report a thymoma that developed 12 years after CABG in a 65-year-old woman. The patient had undergone percutaneous coronary intervention (PCI) with stenting of the left anterior descending( LAD) artery, 13 years ago, and a 4-vessel CABG, 12 years ago. Three-dimensional(3D) coronary computed tomography (CT) and angiography before surgery showed that all grafts were intact, but LAD artery occlusion was presented just distal to the stent. Since the thymoma was located in front of the ascending aorta and trunk of the pulmonary artery, abutting the left and right internal thoracic artery and radial artery graft, we performed PCI for LAD recanalization preoperatively, and complete resection of the thymoma with redo sternotomy was performed safely using an ultrasonic stethoscope without damage to the graft.