The aim of this paper is to show a new geometrical specimen in fatigue to define the behavior of semi elliptical crack growth in thick-walled pressure vessels. This research emphasize on the importance of the behavior of fatigue crack in test specimen and real conditions in vessels .In other word, there must be more adaptations between stress intensity factor in new specimen and real vessel that includes crack along the wall, so, under Fatigue loadings, as a result, more adaptation will be obtained between growth speed and relative crack lifetime according to loading cycles. By introducing this new specimen we compare the results of fatigue loading on propositional new specimen with results of vessel fatigue loading and standard specimen. We estimate the behavior of fatigue crack growth in specimen and pressure vessel with (FEM) and experimental method. In order to comparing of limit component with experimented results, some piece of with new specimen made of steel (ck45) can be used and all of these must be under fatigue testing and results are obtained in these methods.