Introduction: Laparoscopic pyeloplasty (LP) has been widely used in the treatment of pediatric ureteropelvic junction obstruction (UPJO). However, no prior reports with a large pediatric series have focused on the analysis of complications and impact factors of the outcomes. We hypothesized there were risk factors of higher Clavien grade postoperative complications. Objective: To analyze the characteristics of complications and risk factors of high Clavien grade postoperative complications. Patients and methods: All children with UPJO treated with primary transperitoneal LP between July 2016 and July 2018 were retrospectively reviewed. The Clavien complication grades in groups with different weight, intraoperative complication (drainage methods), anteroposterior pelvic diameters (APPD), side, gender, title of surgeon, preoperative presentation and obstruction reason were compared. Results: Of the 279 children, intraoperative complications in which the placement of double-J stents was not accomplished and conversion to open surgery (Satava grade II) occurred in 17 (6.09%) and 2 (0.72%) patients, respectively. A total of 270 patients (277 kidneys) were included in the analysis of postoperative complications. Postoperative complications occurred in 51 (18.89%) patients. The most frequent postoperative incident was febrile UTI in 27 patients. Of the 13 patients who required reoperations, 6 patients had kidney restenosis and were considered as failure of surgery. All complications with an exact onset time occurred within 10 months after surgery. In the univariate and multivariate analysis, weight <10 kg and having intraoperative complication with nephrostomy tube were risk factors of higher Clavien postoperative complication grade (P＜0.05). Discussion: Patients who were <10 kg in weight and having intraoperative complication with nephrostomy tube had a greater risk of a higher Clavien grade postoperative complication. To reduce high Clavien grade postoperative complications, asymptomatic patients under 10 kg in weight and having intraoperative complication with nephrostomy tube need close monitoring. In the 26 patients who had an exact time of the postoperative complications, the longest time we found was 10 months. Thus, we recommend the follow-up time required to observe postoperative complications in patients should be at least 10 months after surgery. Conclusions: LP has been proven to be safe and effective in children with a low rate of complications. Weight <10 kg and having intraoperative complications with nephrostomy tube were risk factors of higher Clavien grade postoperative complications. Children with low weight and intraoperative complications need more attention in terms of the occurrence of complications. [Table presented] © 2020 Journal of Pediatric Urology Company