API579-1/ASME FFS-1 Fitness-for-Service Part 4 and Part 5 Lv.1 and Lv.2 as well as WES 2820 metal loss assessment procedure requires a certain dis-tance from a major stress discontinuity. It limits the FFS assessment often, however, the definition of major structural discontinuities shown in API579-1/ASME FFS-1 Part 4 and Part 5 as well as WES 2820 metal assessment is qualitative. Also, current requirement is based on Timoshenko’s model that assumed an external force and moment working on the surface of cylinder while some of “major structural discontinuity” would not take a significant force and moment. Improving the definition of “major structural discontinuity” would increase the application slot of API579-1/ASME FFS-1 Part 4 and Part 5 Lv.1 and Lv.2 assessment as well as WES 2820 metal loss assessment. In this paper, the remaining strength of pressure vessels with a locally thin area located around structural discontinuities is obtained and discussed for several typical cases, such as 2:1 elliptical head, hemisphere head and stiffening ring, based on the significance of surface correction factor Ms. The result indicated if the cross section of major structural discontinuity was sufficient and no working force and moment on it, it would not affect the remaining strength of pressure boundary.