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A nonlinear static analysis is recently used in computing the seismic demands for the structure subjected to earthquake excitation in performance-based seismic code. Although a nonlinear time history analysis may provide better estimate on the seismic performance of building structure, approximation methods are still needed in the aspect of practicality and simplicity. In converting a multi-story structure to an equivalent SDOF system, the mode vectors of the multi-story structure are assumed as the mode shape in elastic state regardless of elastic or elastic-plastic state. However, after the yielding made in the structural elements, the characteristics of displacement mode are also changed because the structure becomes inelastic in each incremental load step.In this research, a method of converting MDOF system to ESDOF system is presented by using nonlinear displacement mode considering the mode change of structures after the yielding. Also, the accuracy and efficiency of the method are examined as the displacements at the roof level of the multi-story building structures estimated on a basis of this converted displacement response of ESDOF and the method is compared with the displacement at the roof level through the nonlinear dynamic analysis of the multi-story building structures subjected to an actual earthquake excitation.