Uncertainties in seismic demand and structural capacity create conservatism in acceptance criteria for structural performance levels. In case of such conservations, rehabilitation costs can increase, and seismic evaluation results of the structures may be incorrect. In design and rehabilitation of structures, considering uncertainties and reducing them can decrease existing conservations and result in optimal design. Seismic rehabilitation guidelines use reliability index coefficients to consider the uncertainties of the existing structure. In seismic rehabilitation of structures, the existence of uncertainties in the secondary system, which are added to seismic rehabilitation of the existing structure, can lead to an increase in the existing uncertainties. Therefore, in this paper, on the example of rehabilitation of a 3-story steel frame structure of the SAC project by a steel shear wall, quantifying of uncertainties of steel shear walls was considered, and a parametric study of the reliability index of the structure was done. The studied structure was modeled in OpenSees software and was analyzed in the presence of uncertainties of the steel shear wall before and after rehabilitation. Based on the performed analysis and considered uncertainties, values of the reliability index of the rehabilitated structure by the steel shear wall were calculated. According to results, rehabilitation of structure reduced the maximum inter-story drift ratio and the probability of failure, while consideration of uncertainties of rehabilitated structure increased the maximum inter-story drift ratio and the probability of failure, and therefore existing conservatism dropped.