This paper investigates the temperature field effect on the stress state of the turbine rotor blade lock joint elements where breakdowns were observed. The turbine rotor blade joint, when heat is supplied from the steam flow, is in conditions of uneven heating. In this case, the physico-mechanical properties of materials change, and one can observe gradients of the temperature causing unequal thermal expansion of individual parts of the structure. This leads to temperature stresses, which, in combination with mechanical stresses from external loads, can cause significant plastic deformation of the structure, cracks, or damage to structures. To clarify the distribution of structural stresses in the lock joint structure, a problem is solved taking into account the temperature field. The problem is solved in a thermal contact setting, with taking into account the heat transfer influence on the transfer of forces in the lock joint. The contact interaction problem is essentially nonlinear, and the temperature problem is connected with the mechanics problem through previously unknown boundary conditions in the contact. The stress state and the nature of the contact interaction depend on the temperature field, which is determined by interaction conditions. The solution to the thermal contact problem in the lock joint is based on the application of the contact layer model. Zones of expected contact interaction are represented by contact elements. The mechanical interaction of contact surfaces is determined by their mutual penetration. The problem is solved using the finite element method, the total number of elements being 371 498. In this model, there are several zones of contact interaction: namely, the area of contact of the pins with the disk, as well as with the locking blade and adjacent blades; the area of contact of the pressure pads of the roots of adjacent blades and the disk shaft end. In the contact zones, the mesh is thickened. The calculation results are presented in the form of the temperature distribution over the lock joint. It is shown that there is a temperature drop along the radius and width of the disk. The temperature of 533 °С from the side of the steam inlet drops to the level of 525 °С from the side of the steam outlet. Results of the calculated assessment of the stress state of the lock joint of turbine rotor blades are given for the first stage of the medium pressure cylinder of a steam turbine. These results indicate significant stresses that can cause plastic deformation.