For the purpose of reducing computational cost, this paper presents an extension of the Proper Orthogonal Decomposition method (POD) to nonlinear dynamic analysis of reinforced concrete multistory frame structure. The material nonlinearity, due to plasticity and damaging, is modeled by the multifiber section approach. An initial expensive full model nonlinear time-history analysis is conducted for the frame structure subjected to a portion of an initial seismic base excitation. Then, POD modes are extracted and used to reduce the dynamic model for the rest of the initial base excitation or for multiple different ones. By comparing the full and reduced models, the computational cost was lowered by up to 94 % while maintaining good level of accuracy.