Phyllodes tumors are relatively rare neoplasms, accounting for less than 1% of all breast tumors and most of them are diagnosed as benign tumors on histological examination. We report a case of giant phyllodes tumor of the left breast which revealed high tumor marker lebel (CA15-3, CEA, BCA225) in serum. A 44-year-old woman was admitted to the hospital because of a giant tumor of the left breast. Blood examination showed elevated lebels of CA15-3, CEA and BCA225. Besides these elevated levels there were no malignant findings, with a diagnosis of phyllodes tumor the patient underwent a simple mastectomy. The excised tumor was 2838g in weight. Histologically, the most part of the tumor was benign, however a small part was malignant. Furthermore, an atypical epithelium resembling papillotubular carcinoma was shown in some part.