The influence of frictional coefficient on the cylindrical plastic steel specimen under compression is analyzed. The considered methodology allows determining effective stress dependence on effective strains by compression testing. It is found that the reason for the barrel-shaped plastic steel compressive test specimen form is only friction between contact surfaces of tested specimens and supports. The standard methodology suggests using various friction reduction technologies to reduce the friction effect on stress-strain diagram determination. These can be special lubricants or changes in the shape of the specimen bearing surfaces. But all approaches to the issue with stress-strain diagram determination are unable to be implemented under temperatures above 300°C due to not being able to use lubricants. The plastic steel compression process experiment behind the model with varying only friction coefficients is suggested in order to determine the effect of friction coefficient on the shape of a compressed specimen. This approach makes it possible to search into the effect of friction coefficient and compare the results with a full-scale experiment. The plastic steel compression diagram, having been obtained from a full-scale experiment, permits to determine effective stress-strain relationship, which, in turn, allows calculating the constitutive equation coefficients of the material model necessary for carrying out simulations of plastic steels deformation under both room indoor temperature and temperatures up to 900°C, including the plastic deformation influence. The proposed methodology also provides the possibility to obtain an effective stress-strain relationship and varying the friction coefficient between the specimen end surface and the supports being exposed to elevated temperatures, which cannot be treated by standard test methods. It is shown that the initial data for obtaining the effective stress-strain relationship are the difference in specimen diameters and the force-displacement diagram.