Atomic mechanisms of structural transformations leading to fragmentation in vanadium under deformation in constrained conditions without changing its volume are investigated on the basis of the molecular dynamics method. The process of formation of a fragmented structure in a deformed specimen can be divided into two stages. At the first stage, twins nucleate and grow in the crystallite. In the second stage, the orientation of lattice in twins may change due to the intersection of twins leading to their anisotropic deformation. In this case, the directions of stretching and compression of the crystal lattice in the deformed twin quite closely lie in the directions of stretching and compression of the whole crystallite.