A 56-year-old woman was admitted to the hospital because of a large tumor in the right breast. The tumor was 13×10cm in size, with elastic firm consistency. With a suspicion of phyllodes tumor, an excision of the tumor was performed and it was confirmed histopathologically. Simple mastectomy was added because of a small malignant component in the tumor with H-E stain. The tumor was also positive for both CEA and p53 with immunohistochemical stain. Elevated serum CEA level of 9.27ng/ml before operation was reduced to 5.98ng/ml in one month after the removal of the tumor. Malignant phyllodes tumors of the breast are relatively rare. Of these tumors, that with positive CEA stain has been reported to be prone to recur. Strict postperative follow-up would be mandatory for the patient.