The fragility curves are used to indicate the probability of damage to buildings during the earthquakes with different intensities. The main objective of this study is to present a new design procedure for exploring the effectiveness of using viscous dampers in the seismic rehabilitation of steel structures using the seismic fragility analysis. In addition, a straightforward formula is presented to determine the effectiveness of seismic rehabilitation of steel structures with viscous dampers. The inter-story drift as one of the effective parameters for assessing the safety of buildings after an earthquake is used in this study as a damage index. To compare the damage probability of buildings with and without damper, three 3, 9 and 20-story benchmark buildings are used. Furthermore, the earthquake records corresponding to three levels of seismic hazard are used for the nonlinear dynamic analysis. Comparison of the fragility curves between the steel structures controlled and the buildings without the damper shows a significant decrease in the damage probability after the rehabilitation with the viscous dampers. The results of numerical analysis showed that although seismic rehabilitation in all benchmark buildings led to the improved seismic performance, the decrease in the response of the 9-story structure is higher compared to the other buildings.