Steel pipes with different cracks may be unsafe if the geometry of crack is larger than a specific size. The failure assessment diagrams are widely used to investigate the safety of such cracked pipes. These diagrams include three levels, in each of which the area under the failure assessment curve is the safe area, where as the outer are is unsafe. In this study, the API X65 gas transmission pipe with 48 inches outside diameter (commonly used in Iran's energy transmission lines) is investigated under constant internal pressure of 7 MPa for the first time in Iran. In order to analyze the critical crack length, a pipe with fixed crack depth of 8 millimeter and crack lengths of 100, 130, 160, 190, 220 and 250 millimeter is considered. Similarly, for analyzing the critical crack depth, a pipe with fixed crack length of 150 millimeter and crack depths of 3, 5, 7, 8, 9 and 10 millimeter is assumed. Using ABAQUS modeling and standard BS7910 equations, the location of loading points is investigated in failure assessment diagram. The results indicated that, in the case of fixed crack depth (8 millimeter), the critical crack length was 220 millimeter. For the case of fixed crack length (220 milimeter), a critical crack depth of 9 millimeter was obtained.