Jack-up type oil rigs are consisted of legs and platforms. The number of redundancies of legs are generally smaller than those of platforms. As a structure becomes less redundant, reserved strength after the initial yielding tends to decrease. So it is important to clarify the ultimate strengths of tubular joints which are leading causes of overall failures of the legs. In the present paper, the authors carried out theoretical and experimental investigations into the local strengths of T-joints of circular tube. Main conclusions obtained are as follows; 1) For the theoretical elastic-plastic analysis, the plastic node method was applied and its effectiveness was demonstrated. 2) Effects of the thickness and yield stress of the leg plate, diameters of brace and chord, stiffening ribs, leg guide, axial force exerted on chord and clearance between chord and leg guide upon the local strength of T-joints were clarified. 3) Simple formula was proposed for the ultimate carrying capacity of T-joints.