Recently the plastic design method has been brought into the structural design field in order to overcome many difficulties and defects caused by the so-called elastic design. In this paper, the authors, therefore, studied the plastic design method to analyse the test results of bipod-type mast models with cylindrical sections. Experiments were carried out on three series of specimens, i. e., B-series (plane frames) loaded in the plane of the specimen, T-series and P-series (space frames) loaded perpendicularly to the plane. In the latter case, both bending and twisting couples should be taken into account in the calculation. The followings were obtained conclusively; (1) Theoretical collapsing loads (ultimate loads) coincided well with the test results of both plane and space frames. (2) The mechanisms (collapse mode) predicted by the plastic design showed satisfactory agreement with those of experiments and consequently the applicability of plastic design to space frames as well as plane frames was confirmed. (3) In the case of bipod-type masts, it was found that the most economical value of μ (the ratio of full plastic moment at the top of mast to that at the bottom of mast), from the weight-saving standpoint, was approximately 0.4.