The kinetics of many microstructural changes during processing or heat treatment are engaged with diffusion-controlled phenomena. Thus, the atomic diffusion plays a crucial role in many physical processes. It has been recognized that atomic migration in crystalline solids is linked to crystal defects behaviour. As the simplest case, the atomistic mechanism of self-diffusion at high temperature is based on the diffusion of point defects (mostly, vacancies). On the other hand, the diffusion phenomena at low temperatures (or in case of highly defected crystal) can be governed by the fast migration of atoms along spatial crystal defects: grain boundaries and dislocations. The diffusion inside such crystal defects can be caused by the completely different processes than the usual vacancy migration. This work focuses on atomistic simulation of diffusion processes taking place in bcc crystals (Fe, Nb, Mo) in presence of crystal defects.