Abstarct: One way to strengthening reinforced concrete frxames against lateral forces, is the use of steel bracing. Among the types of bracing systems, concentric X bracing due to high stiffness and simple design, have been used extensively. The behavior of 4, 8 and 12 story reinforced concrete frxame that are strengthening by concentric X bracing by nonlinear static analysis and nonlinear incremental dynamic analysis (IDA) were evaluated. Evaluation of the impact of concentric X bracing on the behavior of reinforced concrete frxames have been made by nonlinear static analysis and incremental dynamic analysis to study the dynamic behavior of the structural system under seismic records. The results of the nonlinear static analysis show that, strengthening of 4, 8 and 12 story reinforced concrete frxames with steel bracing in addition to a significant increase in strength and stiffness of the structure, it will also reduce displacement. The increasing stiffness due to adding bracing frxames in 4 story frxames over 8 and 12 sroties. Compare the capacity curve of the frxame on static analysis shows that at a constant lateral displacement, the increasing in height is reduced structural capacity. The result of this structural system similar curves for the IDA also provided. Finally, a comparison of fragility curves IDA analysis shows that in both IO and CP limit states, a taller structures are more likely to fail.