As an extremely rare disease of neck mass, Ectopic Thymic cyst mostly occurs in the left side and is found in children aged 12 to 13 and infants. 80% of the cases show painless mass and dyspnea, or airway compression can be symptom depending on the size and position of mass. Ectopic thymic cyst is rare that this is diagnosed by clinical symptoms or a radiologic examination and is mostly diagnosed, pathologically after surgery.According to the pathological diagnosis, 90% and 10% of the cases are cystic and solid, respectively. The therapeutic principle of Ectopic thymic cyst is based on surgical complete resection, since this is associated with myasthenia gravis and malignant transformation. According to the acute lymphadenitis impression, the authors applied surgical resection to a 34-year-old man who visited the hospital for the right upper neck mass with pain. The pathological tissue examination result showed that the patient was diagnosed with an ectopic thymic cyst with parathyroid gland and the ectopic thymic cyst of the right upper neck which hasn’t been reported from adults in Korea, was treated. Hereupon, this paper reports this case along with literature review