Abstract: Damping reduction factor is one of the important parameters to determine the design earthquake force for building’s structural seismic design. Employing the displacement response spectra of single-degree-of freedom systems with different levels of damping ratios, the present study investigates the effects of the effective ground motion duration on damping reduction factor. The ground motion durations are associated with 25 Chi-chi earthquake records and harmonic sine wave. The study shows that damping reduction factor decreases with the increasing of the effective duration of the ground motion and the number of cycles of harmonic excitation. A nonlinear multiple regression analysis based on the statistical mean value of the present study is employed, and a modified damping reduction factor considering the effects of ground motion duration is suggested.