The seismic vulnerability of a steel-concrete composite slab-on-girder bridge has been assessed in this paper. Firstly, quantity assessment of the bridge has been done and through an in-depth inspection program all defects were detected. Then based on the purposes of this study appropriate Finite Element (FE) model has been created and the vulnerability assessment of the bridge has been carried out. In between the above mentioned phases some destructive and non-destructive tests were carried out in order to find the mechanical properties of the materials as well as the characteristics of the surrounding soils. It has been tried to use a rational modeling for the expansion joints of “balanced type” as well as for the special steel bearings of the bridge. Two different FE models with different levels of accuracy and different element types have been used to increase the reliability of the models and the FE models of the bridge have been updated by comparing the model results of the above mentioned models. Finally the seismic vulnerability of the bridge has been assessed through the standard procedure of seismic spectral analysis of the bridge. Capacity-demand ratios for all the bridge components and elements have been calculated. The results showed that the bridge have adequate seismic performance against Iranian seismic code (standard 2800). It was found that the bridge has low level of seismic vulnerability. The study showed that the bridge still enjoyed relatively appropriate strength reserve and proper dynamic performance despite some minor cracks and steel corrosion suffering from carbonation and poor maintenance.