This study presents a probability density function (PDF) interpolation technique for the evaluation of seismic fragility curves as a function of the return period. Seismic fragility curves have been developed in terms of various seismic intensities, such as the peak ground acceleration (PGA), peak ground velocity and pseudovelocity spectrum. However, of these seismic intensity measures, the return period of design earthquakes is more useful, as seismic hazard curves are generally represented with the return period of design earthquakes; seismic design codes also require consideration of the return period of the design earthquake spectrum for a specific site. Therefore, this paper focuses on the evaluation of seismic fragility curves as a function of the return period. Seismic fragility curves based on the return period are compared with those based on the PGA as an example bridge. The seismic fragility curves developed in this study make more intuitive sense for the design, retrofit and performance evaluation of bridges.