In order to take charge of the part of horizontal loads, braces are commonly used in the frame structures. Designing elastically these structures, in many cases the calculation becomes rather complicated and moreover it is difficult to decide the optimum scantling of the brace. In this paper the bracing systems of plane and spacial structures were studied by plastic design method to obtain strength of braced frame structures. Experiments were carried out on 16 portal frames and 10 spacial frames. Comparing the results of calculation with the experiments, the followings were obtained conclusively. (1) The test results show reasonably a good agreement with the calculations, revealing applicability of plastic design to bracing structures. (see Fig. 2, 3, 10, 11, 12 and 13) (2) In the case of plane frames, when horizontal loads only, collapsing load of a frame increases proportionally with the sectional area of bracing, on the other hand when both horizontal and vertical loads are applied, a diagram of collapsing load versus the sectional area of bracing may have knuckle points so that the optimum design could be attained. (see Fig. 2 and 3) (3) The bracing systems in the experiments are slightly different from the actual systems, but the same methods of calculation could still be applicable to more complicated frame structures.