Objective: To investigate the effect of the diameter of ureteral access sheath (UAS) used during retrograde intrarenal surgery (RIRS) on operative parameters, perioperative ureteral injury, and ureteral stricture development.Materials and Methods: The study was designed as a prospective randomized controlled trial and included 320 patients who underwent RIRS. The patients were divided into two groups according to the diameter of UAS (9.5F/11.5F [Group 1] and 12F/14F [Group 2]) placed during the operation. At the end of the operation, ureteral injury was checked visually using semirigid ureterorenoscopy and classified according to the ureter injury scale. In the postoperative first year, the control CT urography images were used to observe newly developing ureteral dilatation.Results: There was no statistical difference between the two groups in terms of patient and stone characteristics, operative time, postoperative stone-free rate, and postoperative infection development parameters. In Group 1, 30 (18.8%) of the patients had low-grade and 8 (5%) of the patients had high-grade ureteral injury, while in Group 2, 44 (27.5%) had low-grade and 19 (11.9%) had high-grade ureteral injury (p = 0.013). In the postoperative period, ureteral stricture was found in 5 (1.6%) patients, of whom 4 (2.5%) were in Group 2 and 1 (0.6%) (p = 0.371).Conclusion: The results of our study showed that the use of a 12F/14F UAS in patients who are not previously stented increases the risk of high-grade ureteral injuries; however, despite this increase there is no difference in ureteral stricture formation.