Carcinoma showing thymus-like differentiation (CASTLE) is a rare intrathyroidal neoplasm, probably arising from ectopic thymus or branchial pouch remnants. The tumor was first reported by Miyauchi et al. [1].The clinical and pathological features of this tumor were classified by Chan et al. [2] into 4 groups: ectopic hamartomatous thymoma, ectopic cervical thymoma, spindle ephithelial tumor with thymic-like differentiation (SETTLE), and carcinoma showing thymus-like differentiation (CASTLE). Recently, CASTLE has been designated as an independent clinicopathologic entity of thyroid tumors in the most recent edition of the World Health Organization classification of tumors of endocrine organs[3].To our knowledge, less than 100 cases of CASTLE have been reported in the literature, 45 cases of which (including one of the authors' patient) have been identified in China. We report a new case of this entity and suggest recommendations for diagnosis.