True thymic hyperplasia and ectopic thymus are very rare in children. In embryologic aspect, thymus is distributed around cervical area and ends up in mediastinum. This case is simultaneous thymic hyperplasia of neck and mediastinum. Ectopic thymus in the neck and thymic hyperplasia in the mediastinum in children were reported 2 and 7 cases respectively in Korea. In Clinical aspects, these thymic hyperplasia were presented by mass. So we should suspect these benign condition to avoid unnecessary operation or biopsy. We report a case of true thymic hyperplasia in the mediastinum with ectopic thymus in the neck in a 4-month-old male infant and review the relevant literature. We believe this is the first reported case in the world of true thymus hyperplasia in the mediastinum with cervical ectopic thymus in the neck.