Thymoma is the most common anterior mediastinal mass in adult, and arises from thymic epithelial cells which includes consists various proportions of epithelial and lymphocytic aspect. Thymic epithelial cell arises from the ventral wings of the third and fourth branchial pouches in the embryo, and these cells are considered tumorous condition of thymoma. Surgical resection is main treatment of thymoma. And adjuvant chemotherapy or radiation is considered due to postoperative pathologic diagnosis. We experienced a tumor which located from left anterior neck along superior mediastinum on chest radiograph incidentally and diagnosed parathyroid adenoma clinically. After surgical removal, final pathologic report was thymoma. Here, we present the case with a review of the related literatures.