The paper considers the processes of complex plastic deformation of polycrystalline structural alloys under monotonic and cyclic modes of disproportionate loading. The version of the equations of state of elastic plasticity used in this paper is based on the concept of a yield surface and the principle of the gradient of the plastic deformation rate vector to the yield surface at the loading point. This version of the equations of state reflects the main effects of elastic–plastic deformation of the material for arbitrary complex deformation trajectories. The main attention is paid to the issues of modeling the processes of elastic–plastic deformation of polycrystalline structural alloys for complex deformation processes accompanied by rotation of the main areas of stress tensors, total and plastic deformations. To assess the degree of reliability and determine the limits of applicability of the defining relationships of thermoplasticity, numerical studies of complex deformation of structural steels under monotonic and cyclic loading conditions were carried out: (1) multi-link flat broken deformation trajectories (P–q experiment); (2) proportional and disproportionate to cyclic loading trajectories (P–M experiment). It is shown that the thermoplasticity model with kinematic and isotropic hardening qualitatively and with the accuracy required for practical calculations quantitatively describes the main effects of complex plastic deformation of structures under monotonic and cyclic loading conditions.