We have experienced a case of occult papillary thyroid carcinoma presenting as an anterior mediastinal mass in a 40-year-old man. The CT scan revealed a huge mass behind the manubrium of the sternum but the ultrasound examination failed to detect any lesion and developmental defect in the thyroid. Excision of the mediastinal mass and total thyroidectomy were carried out. Histologically, the mediastinal mass turned out to be papillary carcinoma without any portion of the normal thyroid tissue or normal lymph node tissue and the thyroid gland showed a tiny papillary carcinoma with the diameter of 0.3cm. Although a mediastinal mass as the sole presentation of the thyroid carcinoma is very rare, we suggest that a mediastinal mass should be added to the list of possible metastatic thyroid carcinoma.