Application of flat welded ends with stress relief grooves in high-pressure vessels is a common alternative to use of dished vessel ends. It is well established and follows calculation rules given in codes: EN-12952-3 , EN-13445-3 , or in ASME code . However the calculation rules do not give any definite answer what should be the choice of parameters defining a circular stress relief groove, for example, position of the groove and its radius. Usually the choice of them relies on engineering practice. The present paper clearly shows the influence of this choice on stress concentration in the cylinder-endplate junction area. The results of numerical study are verified in experimental investigations performed for a cylindrical high-pressure vessel.