Background: Phacoemulsification is one of the most frequent surgeries in the world. However, prolonged use of phacoemulsification machines produces reactive oxygen species which will damage corneal endothelial cells. Ascorbic acid has an antioxidant capacity to neutralize oxidative stress in the anterior chamber. This study will investigate the protective effect of ascorbic acid on corneal endothelial cells in patients with hard nuclear cataracts. Methods: This study is a double-blinded randomized controlled trial. Samples will be divided into three groups, and 500 mg vitamin C three times daily (1500 mg/day), or placebo will be received for seven weeks. Clinical characteristics, ascorbic acid, malondialdehyde, and total antioxidant capacity of patients in serum and aqueous humor will be measured before and after intervention and phacoemulsification. Conclusions: Data from this study will reveal the protective effect of oral vitamin C supplementation on the corneal endothelial cells in patients with hard nucleus cataracts. This trial has been registered at ClinicalTrials.gov (identifier: NCT06781970; registered on 17 January 2025). The trial record is available at: https://clinicaltrials.gov/ct2/show/NCT06781970 Copyright: © 2026 Widyawati S et al.