A retrospective observational study was conducted to validate the effect of corneal cross-linking (CXL) on eyes with progressing keratoconus, depending on severity.In total, 45 eyes of 33 patients (age: 23.9 ± 6.8 years, range: 14-42 years) with progressive keratoconus who underwent CXL were enrolled. Examinations were performed at pre-, 1, 3, 6, and 12 months after surgery. In addition to a slit lamp microscopy, measurement of visual acuity, the steepest keratometric value (Kmax), the thinnest corneal thickness (TCT), and the corneal endothelial cell density (ECD) were assessed. Change in Kmax (ΔKmax) was calculated by subtracting the preoperative Kmax from the 12-month postoperative Kmax.CDVA, TCT, and ECD did not change significantly throughout the follow-up period. Kmax was 56.4 ± 7.2 D preoperatively and 54.3 ± 5.6 D at 12 months after CXL (P = .174). The average value of ΔKmax was -2.23 ± 4.31 D at 12 months after CXL. ΔKmax was negatively correlated with preoperative Kmax (ρ = -0.5517, P = .0001), and positively correlated with preoperative TCT (ρ = 0.4791, P = .0012). However, no correlation was observed between Kmax and patient age or the decrease ratio of ECD.The more flattening was obtained after CXL in cases with the more advanced keratoconus. No complication, including corneal endothelial damage, was observed even in advanced cases.