In this study induction of neoplasm in thyroid gland of one postoperative patient with breast cancer was conducted by marine algae (also seaweed, sargassum), which is presented here. A 41-year-old women was diagnosed as her right breast cancer complicated with lymph node metastasis in her right axilla on February, 1999. In June 19, 1999 she was given the combination chemotherapy of vincristine, cyclophosphamide, 5-Fluorouracil and cinobufacini drugs due to two lymph nodes on her right superclavicular following radical mastectomy. During chemotherapy she was also taken the adjuvant treatment of traditional medicine. Traditional medicine consisted of seaweed plant drugs (containing iodine 362,400 ug/kg). As to intermittent maintance treatment the total dosage of seaweed herb was at least exceeded 500 gram. Induction of thyroid tumor (thumb size) was found in June, 2001. A thyroidectomy due to thyroma was successfully performed. Histologically there revealed thyroid tissue without the evidence of metastasis of breast cancer. She had a 5-year survivor. The data indicated oncogenic function of some traditional herbs, and further experience of traditional medicine in treating thyroid disease especially in thyroid cancer.