Abstract: A Buckling-Restrained Brace (BRB) is an energy dissipation device of metal yielding and its advantages include the stable energy dissipation capacity, easy construction and fabrication with the low cost, etc. This paper presents the low-cycle fatigue tests of three partly-welded BRB specimens. The torsional buckling failure modes of the unstrained yield segment at the end of the BRB’s core brace are discussed and the theoretical formula is compared with the experimental results. Test results show that the plastic torsional buckling induces the failure of the BRB, which should be focused on and taken as an important failure mode. The effect of the BRB’s axial deformation on the torsional buckling failure is significant. When the axial deformation is larger, the critical stress of the plastic buckling is decreased and the actual axial stress is increased. Furthermore, the theoretical formula can accurately predict the torsional buckling occurance of an unstrained yield segment.