This research aims to investigate the extent that an incorrect assumption for soil type may endanger the seismic safety of moment-resisting steel frame structures using probabilistic assessment. To this aim, first, a set of moment-resisting steel frame structures were designed for the site soil class C. The examined structures were 3-, 6-, and 9-storey moment-resisting steel frames designed according to ASCE7-16. Then, assuming that the actual soil type had been B, C, or D, seismic vulnerability assessments were performed using OpenSees software. For this purpose, a two-dimensional model of each structure was undergone the incremental nonlinear dynamic analysis (IDA) subjected to far-field, near-field (with pulse), and near-field (without pulse) ground motions of FEMA-P695. To take into account the effect of soil-structure interaction (SSI) on the exceedance probability of the models, the foundations were simulated as flexible elements. The fragility curves were developed for each model under each ground motion record type and accordingly the collapse margin ratio for each model was calculated. The results indicated that within the LS performance level at Sa(Design), site soil class B decreases the exceedance probability slightly but site soil class D tends to increase the exceedance probability significantly, especially as the height increases.