Abstract: A design method for all-steel Buckling restrained braces (BRBs) with a flat plate core is proposed based on its failure modes. First, the maximum bending moment of the outer tube for this kind of BRBs is analyzed, and the formula to ensure the overall stability of the BRB is obtained. Second, based on the interaction force between the core and the outer tube, the local buckling of the BRB is studied. In order to avoid local buckling, the ratio of the width to the thickness of the core plate should be less than a critical value. The results indicate the critical value decreases with increasing ratio of the gap (between the core and the outer tube) to the width of the core plate. A ratio of the width to the thickness less than 8 is required to ensure the local stability of the core.