AIM:To investigate the effect of different pupil diameter on contrast visual acuity in dark environment·METHODS:Pupil diameter and contrast visual acuity from 42 case(84 eyes)before LASIK surgery were measured with multi-functional visual acuity tester(MFVA-100) and Allegro wavefront analyzer,comparing the contrast visual acuity related to different pupil diameter and analysing their relationships·RESULTS:Contrast visual acuity was(0.158±0.097)and(0.374±0.099) respectively when pupil diameter ranged from 4.0mm to 5.5mm on both contrast of 25% and 10%,better than the contrast visual acuity when pupil diameter ranged from 5.6mm to 7.2mm,the difference was significant(P<0.02),the difference of contrast visual acuity in 25% was more evident than that in 10%;There were no significant difference when pupil diameter ranged from 4.0mm to 5.5mm and 5.6mm to 7.2mm on both contrast of 100% and 5%(P>0.1);contrast visual acuity(100%,25%,10%,5%) were decreased with the increase of pupil diameter whether from 4.0mm to 5.5mm or from 5.6mm to 7.2mm,there was a positive correlation between them·CONCLUSION: Different pupil diameters could affect the contrast visual acuity obviously under certain conditions,and influence the night visual performance accordingly.The measurement to the pupil size on dark environment before LASIK surgery is one of the important index affecting corneal refractive surgical procedures.