Objective To evaluate the impact of pupil diameter on controlling the development of myopia in orthokeratology. Methods Forty-nine children aged 7 to 14 years were enrolled in this prospective clinical study. Orthokeratology was performed to correct the refractive errors of these children after measurements of refraction， pupil diameter and axial length. Axial length （AL） and pupil diameter were measured 24 months after lens wearing. The average pupil diameter was calculated and was used as the criterion for dividing the children into ″pupil size above average″ vs ″pupil size below average″. Data were analyzed by a paired t-test， an independent t-test， repeated measured ANOVAs and Pearson correlation analysis. Results AL increased significantly throughout the 24-month observation period （F=67.182， P＜0.01）. The axial growth of subjects with below average pupil sizes and those with above average pupil sizes was 0.43±0.28 mm and 0.24±0.27 mm， indicating a statistically significant difference （t=2.379， P＜0.05）. AL change in subjects with above average pupil diameters was 55.81% of that for subjects with below average pupil diameters. Baseline pupil diameter was negatively correlated with axial growth at the 24-month visit （r=-0.361， P＜0.05）. Linear regression analysis was used between baseline pupil diameter and 2-year axial growth： Axial growth=-0.161× baseline pupil diameter +0.987 （F=7.048， P＜0.05）. The average pupil diameter for subjects with above average pupil sizes and those with below average pupil sizes after 2 years was 3.72±0.53 mm and 3.51±0.37 mm， indicating no statistically significant difference （t=2.543， P＞0.05）. Conclusion The effect of orthokeratology to control myopia would be more beneficial for children with larger pupil diameters than for children with smaller pupil diameters. This may be due to the enhancement of the myopic shift in the peripheral retina.