Objective  To analyze the clinical characteristics and effects of making thin corneal flaps using a modified oval ring in sub-Bowman′s keratomileusis (SBK). Methods  In this prospective case-series study， 36 patients (71 eyes) undergoing SBK had corneal flaps created with a modified oval ring and One Use-Plus （OUP） automatic microkeratome. The transverse diameter， longitudinal diameter， and pedicle width of the corneal flaps were recorded in surgery. Measurements of corneal flap thickness and stromal bed thickness were conducted after surgery using anterior segment optical coherence tomography （OCT）， recording intraoperative and postoperative complications. Data were analyzed with a paired samples t test and Pearson correlation. Results  The average transverse diameters and vertical diameters of the flap were （8.78±0.26）mm and （8.58±0.28）mm (t=－4.635， P<0.01)， respectively. The average pedicle width was （4.11±0.35）mm and the average corneal flap thickness was (111.1±7.6)μm. There were no significant differences between the two eyes of each patient. The corneal flaps were all made successfully on the first attempt and there was no bleeding or serious intraoperative or postoperative complications. All corneal flaps were in good alignment. Conclusion  A modified oval ring makes a cross-oval flap and reduces the incidence of bleeding from laser in situ keratomileusis. It is effective for correcting with-the-rule astigmatism， and has proven to be secure and stable.