Purpose: To evaluate the amount of lens decentration and various factors affecting decentration after orthokeratology lens wear and to observe the effect of decentration on the visual functions. Methods: Two kinds of orthokeratology lenses were fitted to 270 eyes of 135 patients [initial mean refractive error: (-3.98±1.51 )D]. Humphery Instruments ATLAS 990 was used for the computer-assisted analysis of corneal topographical maps. The examination of corneal topography was performed on patients before and after 6 months of wearing orthokeratology lenses. The amount of decentration of orthokeratology lenses was measured by finding the distance between center of optic zone and the pupil center. The factors influencing the amount of decentration were analyzed, including the initial refraction error, astigmatism, keratometry values, corneal eccentricity, and the diameter of lens. Visual symptoms including monocular diplopia, glare around lights were recorded to evaluate the effects of decentration on visual functions. Results: The mean amount of decentration was (0.49±0.34) mm after one night’s wear. The mean amount of decentration after 1 month, 3 months and 6 months was (0.57±0.41) mm, (0.55±0.48) mm and (0.59±0.39) mm, respectively. After one month, the amount of decentration was less than 0.50 mm in 51.1% eyes, 0.50-1.0 mm in 35.6% eyes and more than 1.00 mm in 13.3% eyes. The direction of decentration of more than 0.50 mm was mainly in the temporal quadrant (48.5%). Patients with greater initial astigmatism and smaller lenses showed greater decentration (P<0.05). There was no statistically significant difference in decentration between the two groups with different corneal eccentricities and keratometry values (P>0.05). The amount of decentration was greater in patients who complained of monocular diplopia and glare. Conclusions: The amount of decentration of orthokeratology depends on the initial refractive error, astigmatism and the design of orthokeratology lenses. Improvement in fitting technology and lens design can lead to reduced incidence of decentration and visual symptoms. Eye Science 2005;21:132-135. Purpose: To evaluate the amount of lens decentration and various factors affecting decentration after orthokeratology lens wear and to observe the effect of decentration on the visual functions. Methods: Two kinds of orthokeratology lenses were fitted to 270 eyes of 135 patients [initial mean refractive error error : (-3.98 ± 1.51) D]. Humphery Instruments ATLAS 990 was used for the computer-assisted analysis of corneal topographical maps. The examination of corneal topography was performed on patients before and after 6 months of wearing orthokeratology lenses. The amount of decentration of orthokeratology lenses was measured by finding the distance between center of optic zone and the pupil center. The factors influencing the amount of decentration were analyzed, including the initial refraction error, astigmatism, keratometry values, corneal eccentricity, and the diameter of lens. symptoms including monocular diplopia, glare around lights were recorded to evaluate the effects of dece The mean amount of decentration after 1 month, 3 months and 6 months was (0.57 ± 0.41) mm, (0.55 ± 0.48 After one month, the amount of decentration was less than 0.50 mm in 51.1% of eyes, 0.50-1.0 mm in 35.6% eyes and more than 1.00 mm in 13.3% eyes. The direction of decentration of more than 0.50 mm was mainly in the temporal quadrant (48.5%). Patients with greater initial astigmatism and smaller lenses showed greater decentration (P <0.05). There was no statistically significant difference in decentration between the two groups with different corneal eccentricities and keratometry values ​​(P> 0.05). The amount of decentration was greater in patients who complained of monocular diplopia and glare. Conclusions: The amount of decentration of orthokeratology depends on the initial refractive error, astigmatism and the design of orthokeratology lenses. Improvement in fitting technology and lens design can lead to reduced incidence of decentration and visual symptoms. Eye Science 2005; 21: 132-135.