Abstarct: In hybrid structures consisting of concrete columns and steel beams, known as RCS, using concrete in columns and steel in a bending members, the most appropriate and optimal utilization of the features materials are obtained in a structure. Since the earthquake is the most destructive force on the structures, the study of the seismic performance of common structural systems is absolutely necessary. Although, RCS frxames are designed and implemented around the world for around three decades, but probabilistic assessment of seismic performance of RCS structures had less done, and the connections of these structures have attracted the most attention from the researchers. In this study, the RCS frxame performance was evaluated in 3-dimensional model by fragility curves. For this purpose, 3, 5, and 8 story buildings with RCS frxame were designed in accordance with the ACI 318-14 and AISC 360-10 codes, and then the 3-dimentional frxame were modeled by OpenSEES software, and were subjected to Incremental Dynamic Analyzes(IDA). Also, the pushover analysis on these structures was done and using the results obtained from the pushover analysis, the coefficient of behavior of this frxame (R factor) had also calculated. At the end, by drawing the fragility curves of the RCS intermediate rigid frxame and comparing it’s fragility with the fragility of steel intermediate rigid frxame and reinforcement concrete intermediate rigid frxame, it was observed that the RCS frxame is more fragile in all low, mid and high-rise buildings than other frxames, and the steel frxame performs better in low and mid-rise buildings. But in the high-rise building, the concrete frxame has a better performance. In general, it can be said that due to the complexity of design and implementation of joints, the use of RCS frxames in low-rise structures, especially in areas with high seismic risk, may not be economically viable and the effect of RCS structure features are not very noticeable.