A microstructure and a phase composition of low-carbon steels Fe-Mn-V-Ti-C and Fe-Mo-Nb-V-C subjected to severe plastic deformation by equal-channel angular pressing from various initial states (after normalization, quenching, quenching and the subsequent tempering) were studied using the methods of optical metallography, transmission electron microscopy and X-ray diffraction analysis. The ultrafine-grained structures with the average size of structural elements about 300 nm (ferrite) stabilized by disperse particles were formed under equal-channel angular pressing independently on the initial structural and phase states of steels. The investigated states with the close size of structural elements after equal-channel angular pressing are characterized by various levels of strength properties: the structure formed from initial quenched state possesses the greatest values of 0,2 pct offset yield strength and microhardness (σ[0,2]=1125 MPa, H[μ]=3,7 GPa) in comparison with two other states (σ[0,2] =960-990 MPa, H[μ]=3,1-3,3 GPa). The authors have analyzed the major physical factors defining a character of ultrafine-grained structure of low-carbon steels after severe plastic deformation and have estimated the contribution of the main hardening mechanisms into steel yield strength.