Atomic mechanisms of the beginning of plastic deformation and failure initiation in nanoparticles of b.c.c. transition metals are presented in this report. It is shown that strength level of nanoparticles of b.c.c. transition metals is pre-determined by the lattice instability within the local region of the crystal. At uniaxial tension even at low temperatures perfect crystal becomes unstable to shear („orthorhombic“ path), i.e. local shear instability is the main mechanism of stress relaxation in nanoparticles of b.c.c. metals. Specific features of local instability of nanoparticle under hydrostatic tension are considered. A model of the temperature dependence of strength is offered. It is shown that nanoparticle strength decreases as square root function of temperature with temperature growth. Just this is essential difference of the temperature dependence of nanoparticle strength from the same for “ordinary” single- and polycrystals.