Medullary thyroid carcinoma and papillary thyroid carcinoma are different subtypes of thyroid carcinoma. The concomitant occurrence of medullary thyroid carcinoma and papillary thyroid carcinoma as a collision tumor is rare. We describe five cases of medullary and papillary thyroid carcinoma as a collision tumor. Four women and one man underwent thyroidectomy for treatment of thyroid cancer. Collision tumor was then detected by histopathologic finding. Genetic testing, point mutation of the BRAF gene or mutation of the RET gene was performed in three cases. However, only one case had point mutation of the BRAF gene. Exact diagnosis of this uncommon event is important because the strategies for treatment of papillary thyroid carcinoma and medullary thyroid carcinoma are different.