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To analyze the changes in visual performance and optical quality after orthokeratology (ortho-k) treatment and its correlation with the magnitude of lens decentration and the diameter of treatment zone (TxZ).A prospective study was conducted in 27 eyes of 27 patients wearing ortho-k contact lens. Area under log contrast sensitivity function (AULCSF), modulation transfer function cutoff values (MTFcutoff), Strehl ratio (SR), and objective scattering index (OSI) were measured and compared before and 1 month after ortho-k lens treatment. Refractive sphere, astigmatism, and diameter and decentration of TxZ were tested against the change in AULCSF, MTFcutoff, SR, and OSI after ortho-k treatment in stepwise multiple linear regression models.Objective scattering index significantly increased and AULCSF, MTFcutoff, and SR significantly decreased after ortho-k lens wear (all P<0.001). The mean TxZ diameter was 3.61±0.26 mm, and the mean magnitude of TxZ decentration was 0.60±0.16 mm. The amount of lens decentration significantly correlated with the change in AULCSF and MTFcutoff (P=0.027 and P=0.005, respectively). The diameter of TxZ significantly correlated with the change in MTFcutoff and SR (P=0.005 and P=0.024, respectively).Visual and optical quality decreases after ortho-k treatment, which can be alleviated by a larger TxZ diameter and a smaller lens decentration.