The elastic-stress state of internal high pressure (HP) casings of 300 and 500 MW steam turbines is estimated using a three-dimensional computational structural model. The internal molded HP casings, which have a complex spatial form and work under conditions of complex power and thermal loading, are some of the most responsible and expensive steam turbine elements, limiting turbine resources. The simplified computational models used in engineering practice did not allow us to evaluate a number of factors determining stress-strain state (SSS) peculiarities. To clarify the distribution of stresses across the structure of internal HP casings, the SSS problem is solved in a three-dimensional setting with taking into account both the operating conditions and contact interaction of flanges. To determine the degree of influence of individual factors on the SSS, the factors are taken into account sequentially. At this stage, the SSS problem of the internal HP casing is solved in an elastic setting, without taking into account the influence of temperature stresses and deformations. The solution to the contact problem in the flange connections of internal HP casings is based on the application of the contact layer model. Probable contact zones are represented by contact elements, the mechanical interaction of contact surfaces being determined by their mutual penetration. The problem of determining the SSS of the internal HP casings of the K-325-23.5 and K-540-23.5 turbines in a three-dimensional setting is solved with using the finite element method (FEM), the total number of elements being 19,553 and 1,780,141, respectively. The created finite element (FE) models take into account the contact interaction of the flanges of the two casing halves in the horizontal connector zone. In the contact zones, the mesh thickens. Results of the calculated estimation of the SSS of the internal HP casings of 300 and 500 MW steam turbines are given for the elastic deformation, taking into account the influence loads arising during the installation and operation of the turbines.