Purpose: We aimed to investigate whether the angle created by the most lateral portion of the lower ureter is responsible for the difficulty in ureteral access sheath insertion. Methods: All male patients scheduled to undergo flexible ureteroscopy were enrolled. Patients with ureteral stricture in any course of the ureter and patients with problems at the ureteral opening were excluded. We set the coordinates of the most lateral portion of the lower ureter as M (x, y, z) using computed tomography. The bladder outlet was marked as the zero point O (0, 0, 0). The vector that passed through the urethra and point O was expressed as Vuo (0, 0, − u). The vector that passed through points O and M was expressed as Vom (x, y, z). Thus, an angle (90° − α) could be calculated between Vuo and Vom using the rotation angle formula. Results: A total of 671 male patients were enrolled, among whom the ureteral access sheath could not pass through the most lateral portion of the lower ureter in 14 patients. The mean values of the α angle were 38.56° and 48.48° in the patient and control groups, respectively, compared with the 42 age- and sex-matched control subjects (p < 0.001). Conclusion: The ureteral angle determined by the level of the bladder outlet and the most lateral portion of the lower ureter could be a causative factor for the failure of ureteral access sheath insertion.