The developed mathematical description of heat treatment of the crushed wood in rotary drum dryers, and also the results of the mathematical description of the studied process is presented in the article. The results of mathematical modeling have shown that with increase in the diameter of a drum by 3 times its length can be reduced almost by 7 times due to the increase in the relative stay time of particles in flight. It provides the best interaction of particles with the heat carrier. Increase in speed of rotation of the drum twice also allows reducing its length almost by 3 times, but at the same time energy consumption increases and crushing of raw wood materials is observed. It was established that the practical speed of the heat carrier is 0.75-2.6 m/s, which limits the drum length to no more than 8-10 meters.