Anaplastic carcinoma of the thyroid(ACT0 is a rare subtype of thyroid neoplasm. This tumor represents approximately 5-10% of all thyroid malignancies and has poor prognosis. ACT often arises in a long-standing thyroid nodule and has been documented to be associated with a variety of more with a variety of more well-differentiated thyroid carcinomas. We experienced a case of anaplastic thyroid carcinoma who had had about a year history of thyroid goiter. The patients had been injected with sclerosing agents in treatment of preexisting goiter. The ACT in this case had varied cytologic and histologic appearances pleomorphic, giant cells, spindle and squamoid. Immunohistochemically, strong cytoplasmic positivity for cytokeratin was seen in all kinds of tumor cells. Ultrastructurally. the evidences of epithelial differentiation were seen such as intercellular junctions and tonofibrils.