There are a few methods for seismic vulnerability assessmentof bridge structural systems. The development ofvulnerability information in the form of fragility curves is anapproach when the information is to be developed accountingfor a multitude of uncertain sources involved, such as,structural characteristics, estimation of seismic hazard andsite condition. Inherent randomness is involved to theseuncertain factors. The major effort of this study is placed onthe development of empirical fragility functions by utilizingthe damage data from past earthquakes consideringcomposite inherent randomness of the uncertain factors. Theintroduced procedure is suitable for the development offragility curves under the assumption that they can bepresented by two-parameter lognormal distributions. Theempirical fragility curves are developed using 768bridgedamagedata obtained from the 1994 Northridge and the1995 Kobe earthquakes, which could be used as referencecurves for seismic risk assessment studies of the similarbridges in Iran.