A mathematical model for description of the fractional composition and the interphase contact surface during extraction of raw plant materials in a rotary-pulsation extractor (RPE) has been developed. Differential and integral functions of the mass distribution of raw plant materials with respect to the particles size are calculated, and an expression for the surface area of particles in various fractions upon each RPE stage are obtained. Good agreement between test and calculated results was obtained for the experimental extraction of Leonurus cardiaca herbs with soybean oil. The proposed mathematical description of the process of raw plant materials comminution in an RPE makes possible evaluation of the fractional composition of raw plant material and the contact surface of phases at each stage of extraction and for every passage of raw material through the apparatus.