It is important to investigate failure mode and ultimate strength of piping components in order to evaluate seismic integrity of piping. Many failure tests of a thick wall and a high pressure piping for Light Water Reactors (LWRs) have been conducted, and the results suggest that the failure mode which should be considered in the design of a thick wall piping for LWRs under seismic loading is low cycle fatigue. On the other hands, the piping in Sodium cooled Fast Reactors (SFRs) is a thin wall configuration compared to the piping in LWRs. Failure tests of a thin wall piping is necessary because the past failure tests for the piping in LWRs are not enough to discuss failure behavior of a thin wall piping. This present work investigated the failure mode and the ultimate strength of a thin wall tees, the critical parts in seismic evaluation of the actual piping of SFRs.