The paper presents the transmission electron microscopy (TEM) investigations of 0.34C-1Cr-1Ni-1Mo-Fe steel after electrolytic-plasma nitriding at a voltage of 550V during 5min. TEM investigations of thin foils are carried out on a EM-125 microscope and involve two states of the specimen surface: before nitriding (original state) and after nitriding. It is shown that nitriding considerably modifies the phase composition and a list of phases present in steel specimens. In the original state, the specimen structure represents lamellar perlite, ferritic-carbide mix, and fragmented ferrite. After electrolytic-plasma nitriding, the structure comprises lath martensite, α-phase lamellae with colonies of thin parallel lamellae of the γ-phase and coarse grains of the α-phase containing multidirectional γ-phase grains different in shape and size. Layers of residual austenite (γ-phase) are observed on the boundaries of lath martensite, which contain Fe3Mo3N particles. Within these laths there are particles of alloyed cementite M3C and carbonitride Cr2C0.61N0.39. The nitride particles Fe3Mo3N are also observed in all other structural components of nitride steel. It also indicates that electrolytic-plasma nitriding does not change the dislocation structure type but increases the scalar density of dislocations half again. Unlike the original state, the dislocation structure of the three structural types is polarized. The mean value of the excess dislocation density is 2.8 × 1010 cm-2 that is less than that of the scalar dislocation density. The amplitude of internal stresses is found to be 335 MPa.