In the article there are presented the mathemathical model of grain classification process in lamella classifiers oriented for calculation of size composition of coarse and fine material basis on known size composition of feed. There are assumed, that grain composition of feed is described of the log-normal and Rosin-Rammler-Benett function. The calculation of Tromp function 71(d), critical grain size d(50) and sharpness indicators are presented. The model was verified experimentally in the laboratory tests with positive results.