A 74-year-old male was admitted with an abnormal mediastinal shadow. Computed tomography (CT) and magnetic resonance imaging (MRI) of the thorax showed an anterior mediastinal mass without invasion to the ascending aorta and pulmonary artery. In addition, serum gastrin-releasing peptide precursor (Pro GRP) was increased (60.6 pg/ml, normal range <46 pg/ml). Video-assisted thoracoscopic biopsy demonstrated that the mass was thymic carcinoid. Therefore, median sternotomy was performed to facilitate thymectomy, including the tumor with partial resection of the left upper lobe and pericardium. The patient received mediastinal irradiation postoperatively. The postoperative serum level of Pro GRP decreased to the normal limit 6 months later. Although a biological relationship between Pro GRP and thymic carcinoid was not proven, it might be useful marker for detecting tumor recurrence.