A 63-year-old woman with a synchronous huge colorectal liver metastasis was referred to our institution.The lesion was technically diagnosed unresectable because the estimated future remnant liver volume was insufficient due to the invasion of the three hepatic veins and hepatic hilum.She underwent 7 courses of mFOLFOX6 and 14 administrations of cetuximab as conversion chemotherapy.Periodic abdominal CT scans revealed the tumor becoming PR, and she was free of cancer invasion to the left hepatic vein.After the remainder of chemotherapy lasting 4 weeks, right trisectionectomy and combined partial resection of the inferior vena cava and primary closure was performed.The postoperative course was uneventful and the patient was discharged at 20 days after the operation.She underwent chemotherapy postoperatively, and then underwent laparoscopic sigmoidectomy.A conversion chemotherapy using cetuximab may contribute to ward rapidly reducing tumor size and improving the resectability of initially unresectable huge colorectal liver metastases, thus leading to prolonged survival.