AIM:To observe the safety and predictability of excimer laser surgery by measuring the thickness of corneal flap,and to provide help for clinic.METHODS:The operative eyes were divided into 4 groups according to the preparation types of corneal flap.Fifty-five eyes were treated with laser in situ keratomileusis with 90μm femtosecond laser flap preparation (group A).Sixty-one eyes were treated with laser in situ keratomileusis with Moria-Ⅱ 90 microkeratomy flap preparation (group B).Thirty eyes were treated with laser in situ keratomileusis with Moria-Ⅱ110 microkeratomy flap preparation (group C).One hundred and five eyes were treated with laser assisted subepithelial keratectomy (group D).Ultrasonic pachymetry was employed to measure the flap thickness of these four groups.The results were analyzed with t-test or linear-regression by SPSS software.RESULTS:The average thickness of these 4 groups were 90.95±10.82μm (group A),133.61±11.97μm (group B),147.70±16.16μm (group C),66.35±11.95μm (group D).They were compared with t-test and showed statistical difference.Correlations between the thickness and the preoperative parameters were analyzed.Group A showed a weak correlation with the spherical equivalent diopter.Group B revealed a moderate correlation with the spherical equivalent diopter and weak correlation with the preoperative intraocular pressure,the average curvature of the cornea,the stall of the microkeratomy of Moria-Ⅱ.Group D showed moderate correlation with flap preparation,weak correlation with spherical equivalent diopter.There was no statistical difference between the thickness of the flap and the used times of the cone in group A,as well as the used times of the blade in Moria-Ⅱ in group B and C and the eyes either right or left in group D.CONCLUSION:Although the thickness was stable in every group,there was difference in the average thickness of these 4 groups.Spherical equivalent diopter may be one reason for the influence of the corneal thickness.It is more predictable and flexible in the thickness of the corneal flap with femtosecond than with the microkeratomy of Moria-Ⅱ.With the thinnest flap thickness,LASEK is the safest operation