The paper addresses the issue of the structure and phase composition of welded joint and focuses on their investigation after plastic deformation. The contribution of internal stresses to the formation of phase composition and fine structure of welded joint is shown herein. Electrode welding is used to obtain welded joint in St3 steel. Specimens are subjected to a quasi-static tensile deformation ranging from 0 to 5% under 370 MPa loading. TEM investigations on thin foil specimens allow studying the structure and phase composition within the heat-affected zone at 1 mm distance from base material and 0.5 mm from welding material. The degree of plastic deformation is shown for both base and welding materials and includes their morphology, phase composition, defect structure and its parameters. Long-range stresses are divided into plastic and elastic. Plastic deformation has no qualitative effect on the material structure, however, it modifies its quantitative parameters. With the increase of deformation degree, the perlite component becomes more imperfect and transforms, first, to a fractured perlite and then to ferrite, thereby decreasing the volume ratio of perlite. Polarization of the dislocation structure is observed. The amplitude of internal stress fields grows. Unlike the shear stresses, long-range stresses manifest their intensive growth. The elastic component makes the major contribution to the long-range stresses resulting in the formation of microcracks.