This paper presents the results of seismic vulnerability assessment of a three-story steel frame with bolted connections. Accurate fragility curves are essential in the development of a general probabilistic performance-based engineering design. The fragility curves represent the probabilities that the structural damages, under various levels of seismic excitation, exceed specified damage states. In this study, a simplified method is applied to trace the fragility curves. A nonlinear time history analysis is adopted in evaluating the fragility curves for the considered structure. The nonlinear analyses are performed based on two hysteretic models. The first one is a fatigue damage based hysteretic model that allows considering the stiffness degradation produced by the cumulative damage. The second is a hysteretic model, without applying fatigue damage calculation, based on Richard-Abbott model. The performance of the building in terms of maximum inter-story drift ratio is studied at various seismic intensity levels. Finally, the fragility curves of the frame are obtained and compared for both model. Results demonstrate the importance of considering the stiffness degradation produced by fatigue damage. Results also indicate that the responses of the frame and the fragility curves are influenced by low cycle fatigue damage effects.