To describe the operative technique and outcome of one trocar assisted retroperitoneoscopic ureteroureterostomy (OTAU) in 40 cases of complete ureteral duplication in children. From September 2016 to December 2020, 40 patients (12 male and 28 female), less than 10 years of age underwent OTAU. A transverse skin incision of 12 mm in length was created approximately 1 cm above the iliac crest. Muscle was spared and retracted with stay suture to expose the retroperitoneal space. Subsequently, a balloon trocar was then inserted, and pneumoperitoneum was achieved. A 10mm operating laparoscope (Stema, Germany) with a Maryland were used to dissect and isolate the ureters from surrounding tissues. The ureters were then exteriorized and end-to-side ureteroureterostomy was performed using Polydioxanone (PDS) 6/0 running sutures. Patient's demographic, operative and follow-up data were collected prospectively. The mean age of patients was 25.2 months (range: 1 - 105 months). The mean operating time was 81.9 ± 11.3 minutes. There were no intraoperative conversions or complications. After a median follow-up time of 47.5 months, the differential renal function of the pathologic upper pole moiety was preserved in all patients. Ultrasound revealed a significant reduction in upper pole moiety's renal pelvis anterior-posterior diameter from 19.6 ± 9.1 mm preoperatively to 11.1 ± 6.7 mm postoperatively (p<0.05), accompanied by a reduction in ureter's diameter from 10.8 ± 4.4 mm to 4.8 ± 1.2 mm (p<0.05). Overall, all 32 patients with pre-operative symptoms experienced complete symptom resolution. One trocar assisted retroperitoneoscopic ureteroureterostomy is a safe and feasible approach that yields excellent outcomes for complete ureteral duplication. gov Identifier: NCT06350942.