In the previous report, the authors studied the finite element method for large deflection and elastic-plastic analysis of plane frame works. The behavior of the structure was analized by the incremental load procedure (Step-by-Step method). The incremental stiffness matrix of the deformed structure was derived from the principle of virtual work for an incremental theory taking into account the spreading of the plastic regions into members. It is possible to make analysis of the structure including both, material and geometrical nonlinearity by this method. In this report, the authors treat numerical analysis containing the unloading process of plasticity (strain reversal), so that they can practice to determine the maximum strength (ultimate load) of elastic-plastic structures, and also to persue the behavior beyond the ultimate load by using this method. Hitherto, many researchers have dealt with inelastic buckling strength and plastic behavior of compression members. For the first numerical analysis the authors deal with the strength of eccentrically loaded column. with the rectangular cross-section, and compare the obtained results with that of Chwallas. Next, the experiment of H-section columns with small imperfections is carried out to investigate the elastic-plastic characteristics, maximum strength, and behavior beyond the ultimate load of the column. The theoretical and experimental results are in close agreement. Moreover, beam-columns under high axial load and moment. are calculated and discussed comparing with CDC's method and experiments at Lehigh University, relatively to next analysis of framed structures. Secondly, the instability of elastic-plastic frame works is studied. This method is a general method to make it possible to solve easily the complex and multi-story frames, which, so far, have many difficulties in solving and computing the elastic-plastic behavior of unbraced frames including sidesway. To investigate the sidesway failure, the numerical analysis by this method is carried out for portal frames and three storied frame with large vertical loads with no brace, and compared with Prof. Wakabayashi's experiments and that of Lehigh University. There are good agreements between authors numerical analysis and these experiments. The computer program formulated by this method makes it possible to analize 200 elements at most by the use of HITAC 5020 with short computing time. Therefore this method is quite efficient to the analysis of elastic-plastic planar frames involving instability and of determining the ultimate load of framed structures.