Purpose: To evaluate changes in visual acuity, refraction, and keratometric (K) readings after implantation of five millimeters versus six millimeters optical diameter of intracorneal ring segments (ICR) for treatment of keratoconus grade (2-4). Patients and methods: A prospective nonrandomized clinical comparative study. Thirty four eyes were included in this study. They were classified into two groups: Group I: Seventeen eyes of patients with Keratoconus grades (2-4) were subjected to 5 mm optical diameter intracorneal ring implantation. Group II: Seventeen eyes of patient with keratoconus grade (2-4)were subjected to 6 mm optical diameter intracorneal ring implantation. Results: Thirty four eyes of 22 patients (9 males and 13 females) with keratoconus grade (2-4) were enrolled in this study. The mean age + SD of the patients in group I was 26.36 ± 7.06 years and in group II was 23.09 ± 6.92 years. The mean keratometric (K) reading decreased from 50.05 ± 3.64D. to 46.96 ± 4.42D in group I and from50.48 ± 5.18D to 46.91 ± 5.44D in group II. There was a statistically significant improvement in the postoperative uncorrected distance visual acuity, corrected distance visual acuity, K readings, manifest spherical and cylindrical refractive errors, and spherical equivalent in both groups. No serious intraoperative complications were reported. Conclusion: Both 5mm and 6mm ICR improve significantly UCVA, BCVA, decreased the corneal power and corneal astigmatism but 6 mm showed significant improvement in cylindrical errors and BCVA compared to 5 mm ICR.