Steam turbine casings are subject to thermal and mechanical stresses both at full power and during start up. In both situations stresses shall not exceed yield point, and the casing separation flanges shall be tight as no leakage is allowed. Tightness of flanges will also be checked by the hydraulic test that also can be simulated. The 2D models, simplified material stress calculations and large safety coefficients that have been used in the past cannot grant the strength of the structure in any of the above conditions, due to complex casing geometries. Sometimes exceeding thickness will worsen thermal stresses and tightness of flanges. Starting from a draft casing design 3D modeling allows to check stresses and strength also in critical points, to evaluate changes in stresses due to small geometry modifications in critical points, to reduce thickness to a minimum, to avoid possible problems in passing the hydraulic tests, in other words to optimize (by means of trial and error procedure) the design of the casing. An example of “optimization” of the inner and outer casing of a steam turbine is presented.