We report a case of ectopic thymus in the left submandibular area of a two-month-old boy. On US and CT scans,a well-marginated, 3$ imes$2cm-sized solid mass along the left carotid sheath, anteromedial to the sternocleidomastoidmuscle and posterior to the submandibular gland, was seen. CT attenuation of the mass showed that it was similarto that of normal thymus in the anterior mediastinum. Although a rare disease, ectopic thymus should be includedin the differential diagnosis of cervical masses along the carotid sheath in infants.