Sizes of mineral grains determine the rock most important physicochemical properties. However, only qualitative relations have been found. Determination of quantitative relations is difficult due to imperfection of traditional optical microscopy. Use of frequency dependence of ultrasound attenuation is perspective method. The method of experimental determination of frequency dependence of attenuation constant of elastic waves in geological materials by means of laser excitation of ultrasound and the following evaluation of sizes of mineral grains has been developed. Example of its application is given.