The function of corner bracket is considered to reduce the stress concentration at the corner of the main members, to increase the rigidity of the conjunction, etc. In the case where the size of the bracket is comparative to the main members, its contribution to the strength can be expected to a certain extent. In this paper, the buckling, and ultimate strength and the rigidity of triangular corner bracket is theoretically and experimentally investigated with consideration of the effects of welding residual stresses and deflection, and reinforcement by stiffener. For the theoretical analysis, such as elastic-plastic large deflection analysis, thermal elastic-plastic analysis, etc., the methods which were developed by the authors with the aid of the finite element method are applied. Initial deflection of the bracket reduecs the rigidity and ultimate strength as observed in the case of square plates under uniaxial compression. Welding residual stresses in the bracket are not selfbalanced due to the existence of the main members, and are mostly tension. This increases the strength of the bracket. Stiffener provided along the free edge also increases the strength approximately to the yield strength.