This paper analyzed seismic response of SDOF elastic-perfectly plastic (EPP) hysteretic systems considering P-Δ effect. It used totally 370 earthquake records belonging to four site and soil conditions to calculate the inelastic deformation ratios (Cd) with different ductility, and used the characteristic periods Tga and TgR to normalize the Cd spectra to keep the valley spectral values at these two periods. Results indicate that both P-Δ effect and damping ratio have significant effects on Cd , especially nearby the characteristic periods.