Objectives: To report the safety and efficacy of ultra-mini-percutaneous nephrolithotomy (UMPCNL) in flankfree modified supine (FFMS) and prone positions in management of pediatric renal calculi. Patients and Methods: This prospective randomized study included 55 pediatric patients with symptomatic renal stones and suitable for UMPCNL. They were randomized into two groups. Group A included 28 patients who were treated by UMPCNL in FFMS position (with a pad below the ipsilateral shoulder and buttocks, putting ipsilateral upper limb over the chest, and crossing the extended ipsilateral lower limb over the flexed contralateral one) and Group B included 27 patients treated by UMPCNL in the prone position. In both groups dilatation was done to 13F sheath allowing the introduction of 6/7.5F semirigid ureteroscope and fragmentation of stones by Holmium: yttrium-aluminum-garnet laser with a 550-lm fiber laser lithotripter. Results: The operation time in FFMS position UMPCNL group was significantly shorter than prone position UMPCNL group (84.3 – 9.87 vs 99.3 – 8.75 minutes) with p = 0.022. There was no significant difference between both groups in terms of stone-free rate (89.3% vs 88.9%), overall complication rate (including transient fever; 21.4% vs 18.5%), postoperative pain (visual analog scale score; 3.4 – 0.8 vs 3.3 – 0.9), or hospital stay (3.53 – 0.8 vs 4.1 – 1.1 days). Conclusion: Both UMPCNL in FFMs and prone positions are feasible, safe, and effective in treatment of pediatric renal stones with relatively shorter operative time in FFMS position.