It is well known that the strength of reinforced concrete columns is degraded by cyclic loading, particularly in the post-peak region. In examining the seismic safety of RC structures, it is important to conduct dynamic response analysis that considers strength degradation caused by cyclic loading. In this study, we first developed a nonlinear dynamic hysteresis model that can take into account strength degradation. Secondly, we attempted to evaluate the effects of duration time of earthquake motion in the post-peak region through seismic response analysis using the proposed model. The results clarified that the post-peak behavior of reinforced concrete structures varies depending on the duration time of the ground motion.