Background: CT-guided marking technique is rarely used in abdominal or urologic surgery. We developed and performed a marking technique for a small tumor, undetectable by ultrasound, using CT guidance before laparoscopic resection of the tumor. Case Presentation: A 73-year-old woman with a history of breast cancer underwent right colectomy with D3 lymph node dissection for ascending colon cancer. Five years after the operation, a solitary tumor was found in the right pararenal region of the retroperitoneal space on enhanced abdominal CT. The tumor was 20 mm in diameter and undetectable by ultrasound, so we performed a marking technique using CT guidance before the operation. Placing the patient in a prone position on the CT table, a 22-gauge needle was inserted into the Gerota's fascia percutaneously and a mixed fluid containing India ink and Iopamidol was injected para to the tumor by the radiologist. During the surgery, the marker was clearly identified and the cutting line was determined to ensure a sufficient surgical margin. The tumor was laparoscopically resected as planned. The histopathologic diagnosis was adenocarcinoma, compatible with metastasis of colon cancer. The postoperative course was uneventful and the patient remained free of disease at 10 months after surgery. Conclusion: When resecting small tumors or tumors with an irregular margin, a marking technique is conducted before the surgery. But, preoperative CT-guided marking has not been applied generally for resection of intraabdominal lesion yet. CT-guided marking can be effective when performing minimally invasive and curable surgery on small tumors. This is the first report of an effective CT-guided marking before retroperitoneal laparoscopic tumorectomy. We believe that this technique provides an important therapeutic option for small tumors that may be undetectable by ultrasound.