During past seismic events, damages of many existing structures occurred. Retrofit of these structures before the earthquake reduces the hazard and mitigate casualties in the future. In this paper, the seismic assessment and retrofit of Reinforced Concrete (RC) structures not designed to withstand seismic loads due to changing demands is considered. After a brief introduction about seismic retrofit described for existing structures, methods of assessing the seismic vulnerability of existing buildings are presented. To show the effect of the selected retrofit approach on seismic behavior of RC building, an existing fifteen storey building used as a hospital, was selected. Since the demand of the selected RC building designed as residential building, was changed, seismic assessment was done using Opensees software to show whether the building needs seismic retrofit or not. After macro modeling to assess the seismic behavior of the structure, dynamic analysis was done based on Iranian seismic code. Selected retrofit approach, which contains steel jacketing and adding structural elements to increase the capacity of structure by considering innovations in connections, was done based on Iranian codes. The results show that the building needs seismic retrofit because of increasing in demand. Furthermore, new designed elements and connections between beams and columns, shear walls, shear walls and foundation illustrate an effective role in improving the seismic behavior of building under earthquake loads.