The purpose of this thesis is to assess seismic performance of RC buildings retrofitted by external braced frame with consideration of soil-structure interaction. Most of existent RC buildings located in seismic areas have been designed and analyzed by outdated codes. Recently, several new methods have been introduced for improving seismic behavior of RC buildings; one of these methods is retrofitting by external steel frames. The main advantage of this method is less of intervention in operation buildings. On the other hand, taking into account the effect of soil in performance of structure during earthquake has been widely discussed in recent years. The flexibility of soil can influence the performance of the structure during earthquake substantially. Thus far, soil’s effect research has emphasized on seismic site effect more than soil-structure interaction. In this study three stories RC building located on soft soil high seismic vulnerability has been studied. The building has been retrofitted by two external steel braced frames at theirs exterior frames in order to improve the seismic performance to the acceptable level. Soil has been modeled as a beam-on-nonlinear Winkler foundation (BNWF) in ABAQUS software and time-history analysis for several real-world earthquakes has been carried out. The results show that considering soil-structure interaction increases the fundamental period and makes the system more flexible. Besides, the structure response becomes damped to a certain degree and distribution of the forces will be altered.