The medullary carcinoma of the thyroid gland is relatively rare tumor, accounting about 1.2~10% of the all thyroid malignancies, which arises from the parafollicular C-cells in thyroid gland. Operation is the only means to cure the patients. Serial concentrations of serum CEA and calcitonin seem to play an important role in the diagnosis and clinical management and also in the therapeutic monitoring of patients with medullary thyroid cancer. In this case, patient with high serum CEA levels after the resection of rectal cancer underwent abdomen CT scan, colonofibroscopy, FDG-PET scan and chest X-ray, but this imaging methods couldn't detect recurrent evidence of rectal cancer. Neck ultrasonography was performed after 8 years from operation, and fine needle aspiration biopsy was performed for thyroid nodule. As diagnosed to suspicious medullary carcinoma, patient underwent total thyroidectomy and central compartment neck dissection. Patient diagnosed as medullary thyroid cancer without lymph node metastasis and capsular invasion pathologically. As well as this case, in patients with high serum CEA levels after definitive surgical resection of gastrointestinal cancer, if imaging study or FDG-PET scan detect no evidence of recurrence, evaluation of thyroid such as neck ultrasonography and serum calcitonin should be performed.