In the seismic reliability assessment process, many uncertainty sources are usually involved in the analysis. In general, two sets of uncertainty sources can be involved in in the analysis, the first group involved in the hazard computation, the second group involved in the fragility computation. This paper wants to analyse the main uncertainty sources involved in both hazard and fragility analysis, and then investigate their impact on the final reliability index variability. The case study is represented by a multi-span steel-concrete composite bridge and results shows the uncertainty of the final reliability index is influenced by the variability of the adopted models an parameters.