AIM:To compare the calculation method with diagnostic method in determining the power of rigid gas permeable contact lens (RGP), and to notice that diagnostic method is superior to calculation method.METHODS:Sixteen cases(31 eyes) with myopia or myopic astigmatism were fitted by diagnostic method, after wearing optometry combined with vertex conversion to determine the RGP diopter, then using the determined RGP base curves combined with steep add minus(SAM), flat add plus(FAP) principle and the vertex conversion to calculate the RGP refraction.RESULTS:In 31 eyes determined by calculating the RGP refraction, except 2 eyes' diopter were similar with diagnostic method, the other 29 eyes were all overcorrected, more than half (18/31) eyes were overcorrected ≥-0.50D. 30 eyes have corrected VA≥1.0 with RGP fitted with diagnostic method, while only 1 eye has corrected VA as 0.8.CONCLUSION:Calculation method can cause over correction of myopia in determining RGP power. The RGP power determined by diagnostic method is lower than that determined by calculation method.