Context Ultrasound (US)-guided quadratus lumborum block (QLB) is a new abdominal truncal block used for perioperative pain management of both upper and lower abdominal surgeries. Aims To compare the analgesic efficacy of US-guided QLB with transversus abdominis plane (TAP) block. Settings Approval of Zagazig Institutional Review Board (IRB) was obtained (ZU-IRB # 5504/ 16-9-2018) before the study, and patient’s informed written consent was also obtained. Our clinical study was registered with ClinicalTrials.gov (NCT04031196). A total of 34 pediatric patients scheduled for elective laparoscopic appendectomy were randomly allocated in two equal groups: QLB and TAP block groups. Patients and methods All patients received general anesthesia using intravenous 2 mg/kg propofol, 1 μg/ kg fentanyl, and atracurium 0.5 mg/kg. After induction of anesthesia, QLB group received bilateral US-guided QLB type 2, using 0.5 ml/kg of 0.25% levobupivacaine, whereas TAP block group received bilateral US-guided TAP block using 0.5 ml/kg of 0.25% levobupivacaine. The primary outcomes were changes of intraoperative hemodynamics and postoperative pain assessment. Secondary outcomes were total intraoperative fentanyl consumption, the first time of rescue analgesics, total rescue analgesic consumption in the first postoperative 24 h, and degree of parent satisfaction. Results QLB group had statistically significant lower hemodynamic changes 15 min after performing the block to the end of surgery (P<0.05), highly significant lower visual analog scale score in the first postoperative 4 h (P<0.001), highly significant lower intraoperative fentanyl dose (P<0.001), significant longer time for the first rescue analgesic, lower analgesic doses given in the first 24 h postoperatively (P<0.001), and higher parent satisfaction compared with TAP block group. Conclusion This study revealed that QLB provides longer and more effective postoperative analgesia compared with TAP block in pediatric patients undergoing laparoscopic appendectomy. Keywords: appendectomy, laparoscopic, pediatric, quadratus lumborum block, regional anesthesia, transversus abdominis plane