In this study, an improved effective damping (ED) equation is proposed to obtain more reasonable estimates of the actual nonlinear response of seismic-isolated bridges (SIB) using equivalent linear (EL) analysis procedure. For this purpose, first the EL analysis results using AASHTO's ED equation is evaluated using harmonic and seismic ground motions. The effect of several parameters such as substructure stiffness, isolator properties, and the intensity and frequency characteristics of the ground motion are considered in the evaluation. Next, the effect of the superstructure mass on the ED ratio is studied. It is found that the accuracy of the EL analysis results is affected by the frequency characteristics and intensity of the ground motion. It is also demonstrated that AASHTO's ED equation should incorporate the effective period of the SIB and isolator properties for a more accurate estimation of the seismic response quantities. A new ED equation that includes such parameters is formulated and found to improve the accuracy of the EL analysis.