A case of mediastinal seminoma is presented. A male driver, 27 years old Korean, has been suffered from substernal pain, cough and moderate swallowing difficulty since 5 months prior to this admission. At the time of onset, he visited at a local clinic to find some mass in his anterior mediastinum on chest P-A and lateral X-ray check. Recently, intermittent hiccups with much aggravated dysphagia forced him to visit our hospital, and admitted for radical resection under the impression of anterior superior mediastinal tumor of thymus origin. Median sternotomy was done and total resection of the tumor of 8.0X11.0X3.5cm was done without any specific complication and biopsy of the tumor revealed as primary mediastinal seminoma of the mediastinum. Supplementary prophylactic irradiation therapy was done with a tumor dose of 4,000 rad in 4 weeks after operation. Postoperative hospital course was uneventful and patient was joyful with his occupation for 3 months after discharge.[KTCS 1978;2:170-174]