Riedel's thyroiditis is a rare variant of thyroiditis and characterized by the replacement of thyroid parenchyma by extensive fibrosis. Typically, the thyroid is diffusely involved and a painless, hard anterior neck mass shows clinical features similar to those of anaplastic thyroid carcinoma: a rapidly enlarging, hard, fixed thyroid mass and symptoms such as dysphagia, dysphonia, and dyspnea. We experienced a case of Riedel's thyroiditis that had presented rapidly growing, hard, fixed, thyroid mass mimicking anaplastic thyroid cancer in a 41-year-old female patient with longstanding benign thyroid nodule for 6 years. The clinical features were indistinguishable from that of anaplastic transformation and open biopsy could exclude anaplastic thyroid cancer. After surgery final diagnosis of Riedel's thyroiditis could be made by typical microscopic findings and immunohistochemical studies. We reported the case with review of the related literatures.