To index X-ra.y powder diffraction lines of an unknown crystal is, in. general, based on the known cell constants, or on the high symmetry of crys- tals. The constants and λ in the d-calculating formula can be cancelled by the ratio method. The experimental d(A)-values are expressed by the recip- roca,ls of the squares of the (dA)-values, i. e. d*2hkl.Ry. By dividing the rear d*2hkl´s by those of the first, second, third etc., in the order of the initial descending d(A)-values we acquire the ratio values. In order to reduce the calculation, we take first ten or twenty lines in diffraction patterns and take three dividings with the 1st, 2nd and 3rd d*2hkl as divisors. For isometric crystals, there is a maximum common factor (M. C. F.)in the ratio d*2(hkl)/d*2(hkl)m,i.e. a*2( =d*2100).For tetragonal, trigonal or hexa- gontal crystals, the ratios of d*2hoo, d*2oko and d*2hko are either the same as those in isometric or showing some particular values;c can be found after the determination of a and b. For orthorhombic, d*2(hkl)n/d*2(hkl)m=h2n+k2n/(b/a)2+l2n/(C/a)2/h2m+k2m(b/a)2+l2m/(c/a)2 and for monoclinic, d*2(hk!)n/d*2(hkl)m=1/sin2β[h2n+l2n/(c/a)2-2hnlncosβ/(c/a)]+ k2n/(b/a)2/1/sin2β[h2m+l2m/(C/a)2-2hmlmncosβ/(c/a)]+k2m/(b/a)2 Calculations of the ratios show that the more the approach of b/a's and c/a´ s of two crystals, the more the approach of their respective ratios of the d*2hkl´s. The essential of the method is to find the b/a(=A) and c/a(=C) from the given d(A)-values by calculating and comparing the ratios of the d*2hkl (Qhkl).These ratios are arranged systematically with A, C as coordinates in order to facilitate the locating of &C of an unknown. Sixty orthorhom- bic minerals of known &C were calculated and the ratios of the nearest A and C may be discriminated, as in A=0.904，C=0.7,80 &A=0.904,C=0.784， A=0 .8919，C=0.354&A=0.983，C=0.355 and hence the method is practi- cable. The same and high reliabilities of both a,b,c and d(A)-values are neces- sary. The accuracy of calculations depends on that of a, b, c and d(A)-va- lues,found experimentally. For monoclinic crystals the angle varies from 90° to 120°.