Thymoma is an epithelial neoplasm of the thymus, which commonly lies in the anterior mediastinum and, therefore, an intrathoracic origin is considered to be rare. This report presents a case of giant thymoma arising in the thoracic cavity. A 61-year-old male presented with a chronic cough and breathlessness. Chest CT revealed a well enhanced giant mass approximately 18 cm in diameter in the right thoracic cavity. FDG-PET showed that the SUVmax of the tumor was 5.0 in the center and almost 2.5 in the surrounding area. A trans- bronchial needle biopsy was performed to find atypical cells. Surgery was scheduled based on the radiological and histological findings. A well-defined giant mass in the thoracic cavity, measuring 18 × 14.5 × 11 cm had undergone expansive growth without apparent invasion. The tumor was completely resected without combined resection of the other organs. The weight of the tumor was 1350 g. The tumor was histologically diagnosed to be type AB thymoma according to the World Health Organization classification and Masaoka stage IIB.