A 65-year-old male, who had undergone an esophagectomy for esophageal cancer at the age of 53, presented a mediastinal growthing thymic cyst. The computed tomography (CT) and magnetic resonance imaging (MRI) demonstrated a cystic and thick wall in the anterosuperior mediastinum, and fluorodeoxyglucose-positron emission tomography (FDG-PET) showed strongly positive finding. Operation was performed and histological examination demonstrated thymic squamous carcinoma arising from thymic cyst. He underwent postoperative radiotherapy and survives 5 years after operation. As thymic cancer with thymic cyst is very rare, we presented this case with a review of the literature.