Abstract: As to the LBB evaluation performed in China, the materials are cast austenitic stainless steel（CASS） or wrought stainless steel（WSS）. In this paper, LBB assessment in the guidance of SRP 3.6.3 was performed to evaluate the main loop piping lines of CASS and WSS. The adjustment due to thermal aging is performed to achieve reasonable material properties. J integral/tearing modulus approach is used to determine the critical crack size of CASS pipe and limit load approach is used to determine the critical crack size of WSS pipe. Leakage flaw size is determined based on Henry’s homogeneous non-equilibrium critical flow model. In order to demonstrate that fatigue crack growth is not a potential source of pipe rupture for the evaluated piping lines, the fatigue crack growth of a postulated circumferential part-through-wall crack and the fatigue crack growth of a circumferential through-wall crack are analyzed. The results show that WSS material is better than CASS material in LBB capability.