Development of the nanomaterial science heightened the interest in studying the deformation mechanism accompanied by grain boundary sliding (GBS). However, the experimental study of GBS processes in mi­cro- and nanovolumes of metallic materials faces difficulties attributed to high localization of strains and requires the use of modern methods and tools of high-resolution electron microscopy. Therefore, the avail­able literature data refer mainly to theoretical and model studies in this area. Treatment of metal alloys with concentrated energy flows, for example, laser irradiation, can impart characteristic features to the grain boundary slip. In this regard, we performed the experimental study of the microplastic deformation of Armco iron with a single-phase ferrite structure under pulsed laser processing. To exclude the effect of phase transformations on the deformation process, only the heat-affected zone (HAZ) was studied. The temperature in HAZ was below the temperature of the first critical point and did not exceed 700°C, which made it possible to consider the total deformation an equivalent of the GBS deformation. The microstructure studies by the methods of optical and scanning electron microscopy revealed that in condi­tions of ultrafast heating and cooling during laser processing of metal, deformation occurred with the par­ticipation of the GBS mechanism. The characteristic features of GBS, i.e., the presence of stepped bound­ aries and accommodation zones, as well as the appearance of high-angle grain boundaries were observed. A technique is proposed for measuring the strain value through the GBS mechanism under an assump­tion that the strain vector components for cubic lattices are statistically equal. A statistical analysis of the measurements of the orthogonal component of the strain vector using the secant method was performed, which provided determination of the relative strain values by GBS mechanism in Armco iron within a range of 1.2-5.9%.