Abstract: The layout principle of the buckling-restrained braces (BRBs) for buckling-restrained braced frames (BRBFs) significantly affects the mitigating effect and the structural aseismic performance. To test the mitigating effect, 6-story and 16-story steel moment resisting frames are built as benchmark models. Taking the displacement and the drift concentration factor (DCF) respectively as the indices to measure the structural seismic damage degree and the damage concentration effect, the optimal, economic and practical brace layout principle are studied between the braces and the frames as well as those among different stories. The ruselts show that the mitigating effect is weakened with the increase of the BRB area, and the fundamental period of structures shoud reduce no more than 1.5~2.0 times after installing the BRBs to the main frames. When the linear distribution proportional coefficient of the BRB area among different stories is 0.5~1.0, the DCF of the BRBF is insignificant. Using the performance target that the yielding of the upper BRBs should occur before the adjacent lower frame yielding, a theoretical formula determining the upper limit value of the BRB-area-ratio among different stories is deduced. A numerical example is used to verify the formula, showing that when the stiffness or the yield displacement of the main frames is small, the BRB-area-ratio should be limited rigorously to prevent the serious phenomenon of damage concentration that the BRBs yield only at one or a few stories. Based on the above layout principle, the design process of the BRBF is obtained.