In technological processes in forestry it is nec-essary to clear seed materials from foreign impuri-ty. The analysis of technologies of cleaning of seeds of Siberian cedar pine on sieves is carried out. One of the ways of cleaning is sifting of seed material on various (inclined, cylindrical) sieves. On the sieves established obliquely the probability of seed damage in sieve openings decreases. It is shown that for the improvement of the quality of seeds cleaning it is necessary to use the whole surface of the sieve for rational distribution of sifted material on its surface. There is a problem of opti-mization of this technological process. At optimum technological process of separation of seeds from impurity as criteria of the equipment productivity, the quality of division, decrease in losses of seed material when sifting can be chosen. Due to big losses of seed material when sifting on inclined sieves there was a need of developing mathemati-cal model of the movement of horizontal sieve. In the study the movement of horizontal sieve is de- scribed, the description is based on the schedule of speed assuming that the design of the mechanism of the drive of horizontal sieve is used in these re-searches. As a result of conducted theoretical re-search mathematical model of the movement of the sieve when sifting cedar lots is received. On the basis of modeling it is possible to claim that the difference of accelerations in the direct and oppo-site direction will allow setting lots movements on the whole surface of the sieve, to separate seeds from impurity and to reduce losses. The research defined the best modes of the movement of hori-zontal sieve. In practice it will allow to reduce loss-es of seed material and to increase the quality of cleaning. The technique is recommended for the design of the equipment for the purification of seed material of Siberian cedar pine.