In this paper the impact of square read more opening, as a source of stress concentration, on the distribution of stress in parts of machines and devices is considered.The fact is that these forms of the openings occur less in practice, but the knowledge of the influence of the square opening on the stress distribution, from the aspect of its size, the radius of the angles curvature and positions in relation to the attack lines of the external load, is very important for those who are dealing with designing and constructing parts that contain such shapes of openings.Therefore, the aim of this paper is to analyse the influence of the position and radius of the angles powell and mahoney bloody mary mix curvature of the square opening on the stress distribution in parts of machines and devices, made of isotropic materials, because these materials are most often used and common in engineering practice.In order to obtain the results of the stress distribution, analytical and numerical methods were used in this paper.