Plastic deformation of crystalline materials is mostly mediated by the motion of dislocations. It is known that the dislocation motion encounters drag related to the crystal lattice and various defects such as vacancies, impurity atoms, and grain boundaries. Under the conditions of quasi-static straining, the dislocations overcome this resistance due to the joint action of the applied stress and thermal fluctuations of atoms. The action of the thermal fluctuations of atoms on any region of a dislocation has a discrete character, since the time intervals during which the thermal fluctuations of atoms favor the motion alternate with time intervals where this action is absent. In accordance with the laws of statistical physics, this alternation takes place at a very high frequency. For this reason, the discrete character is not manifested under the usual experimental conditions. However, in connection with the investigations of material properties under impact loading of ultrashort duration, which have been conducted extensively in recent years [1], a question naturally arises: how short should the external loading duration be in order for thermal fluctuations of atoms to have, with a large probability, no time to set dislocations in motion during this loading? In this study, it is shown on the basis of the Einstein‐ Debye fluctuation theory and the results of our previous investigations [2] that, during the impact loading of a crystal with a duration of 10 –5 to 10 –6 s and a stress level on the order of the dynamic yield strength, the thermal fluctuations of atoms required for the dislocation motion do not occur (with a probability of nearly unity). As a result, the crystal remains plastically undeformed, though at the same stress level but for more prolonged loading it would experience plastic deformation. This effect can be compensated, that is, the plastic deformation of a sample subjected to a short loading pulse can be achieved, by increasing the stress level during the pulse. This is indicative of an increase in the yield strength of a material with decreasing duration of impact loading. Let us consider a crystal as an elastic isotropic body containing dislocations of one slip system. It is known that the following condition must be fulfilled in order for a dislocation at rest to be set in motion: in the vicinity of the dislocation, the tangential component of the stress tensor acting in the dislocation slip plane in the direction of the Burgers vector must exceed in absolute value a certain threshold σ 0 which characterizes the resistance impeding the dislocation motion. We will analyze a situation in which the external stress component σ B does not reach the threshold level; for certainty, let σ B be positive, so that we have 0 < σ B < σ 0 . In this case, favorable action of the thermal motion of atoms is needed for the onset of dislocation motion.