Thymus is a lymphoid organs with endocrine function . Thymus develops to larger in the neonatal and early childhood, after sexually mature, it will gradually shrink to degradation.Thymus is divided into left and right lobe, asymmetric, adult thymus is about 25 to 40 grams, color gray red, soft, mainly located in the anterior mediastinum.Thymus is hematopoietic organ in the embryonic, in adulthood it can produce lymphocytes, plasma cells, and myeloid cells.Thymus reticular epithelial cells secrete thymosin, it can promote the producing and maturing of T cells with immune function , and it also can inhibit the synthesis and release of acetylcholine of motor nerve terminals.when there is thymoma, thymosin will increase, this could lead to the myasthenia gravis because of emergence of neuromuscular disorder.