Polycrystalline metals have flow stress two to three orders of magnitude lower than the theoretical shear strength estimated by Frenkel model. This significant strength difference is primarily due to the presence of defects, such as dislocations and grain boundaries. However, it was experimentally found that defect-free nanoscale objects (whiskers, nanopillars, etc.) can exhibit strength close to the theoretical limit. With the development of nanotechnology, interest in the study of the theoretical strength of metals and alloys has grown significantly. It is important to find reliable criteria of lattice instability when homogeneous nucleation of defects begins during deformation of an ideal crystal lattice. Note that the Frenkel estimation does not take into account thermal vibrations of atoms and attempts are being made to take into account the effect of temperature on the theoretical strength of defect-free crystals. In this paper, using molecular dynamics simulation, we study shear deformation in the direction of for single crystals of copper and aluminum in the temperature range from 0 to 400 K. Lattice instability was evaluated using two criteria: (i) macroscopic criterion, which is related to the loss of positive definiteness of the stiffness tensor, and (ii) a microscopic criterion related to the formation of a stacking fault, which leads to a drop of the applied shear stress. It was demonstrated that both criteria are consistent at low temperatures, but the macroscopic criterion is less reliable at higher temperatures.