Abstract: A buckling-restrained diagonal brace is provided. The buckling-restrained diagonal brace mainly contains a core element and a restraining element. The core element is an assembly of a specific number of steel members having a specific shape whose cross-sectional dimension is determined based on the load requirement. The core element has a larger cross-section at the ends than at its medial section. The core element is encased in the restraining element. The restraining element manly contains a single or multiple steel tubes filled with concrete, mortar, or reinforcing members. The restraining element is to protect the core element so that it wouldn't be buckled and destructed under load. By using a lapping or an improved plugging type of connection to a structure frame, the buckling-restrained diagonal brace is able to yield steadily when loaded in both tension and compression and the buckling-restrained diagonal brace would have a full hysteretic curve.