For the quantitative analyses of minerals by X-ray diffractometer, commonly used sample type is infinite-thick random powders in terms of penetration depth of X-ray. But on the other hand, oriented samples are used for the qualitative clay analysis. In order to carried out both analyses at once, theory of diffraction intensity from finite-thick sample must be well-understood. For this purpose, theory of diffraction intensity from finite-thick sample has been reviewed and applied to anatase analysis in the oriented clay samples. Suspensions were prepared from various amount of anatase containing samples and air-dried on the aluminum slides. Diffraction lines belonging to aluminum slide beneath the samples were obtained by X-raying the samples with copper-K alpha radiation. Relative mass absorption coefficients of the samples and also corrected intensities of anatase diffraction lines were determined by the intensities of the aluminum diffraction lines. Corected intensities of anatase diffraction lines were plotted against weight fractions of anatase in the samples, and equation of curve was obtained by the least-squares analysis. Result shows that the method is as precise as the internal standard method..