Abstarct: Due to the fact that beam and column joints play an important role in the design of reinforced concrete structures and most of the existing analysis and design software consider the area of beam to column connection in reinforced concrete flexural frxames, so this assumption is difficult. It raises the structure too much and it is not possible to make an¬ accurate assessment of the seismic performance of the structure. Therefore, we want to achieve a more accurate seismic performance evaluation by considering the flexibility in the joints. Calculating the level of performance of buildings with the help of fragility curves is a good way to calculate the amount of potential damage. With the help of these curves, the fragility of different structures is determined and with the help of this data, a suitable solution can be provided to eliminate possible weaknesses in the structures. Therefore, three models of 3, 5 and 8 storey buildings with similar plans and the structural system of moderate bending frxame considered in Itbas software are designed. Then, by modeling them in Appendix software, IDA analysis is performed once, considering rigidity and again, considering the flexibility of connections. With the help of the results obtained from IDA analysis, fragility curves at different failure levels including (low, medium, high and complete failure) are plotted for all models. By comparing these fragility curves and the median values of fragility, the effect of joint flexibility on the seismic performance of buildings can be investigated. Examining the results, it can be seen that in structures whose connection is flexible, the structure soon reaches four modes of failure: low, medium, high and complete. The median values of seismic fragility in the 5-laxyer model under earthquake at partial, medium, high and complete damage levels are 0.08, 0.1, 0.2 and 0.38, respectively, and in the rigid state these values are 0.1, 0.15, 0.26 and 0.44. The numbers are in units of ground gravity acceleration (g), thus reducing the seismic brittleness level of the flexible structure for a flexible structure by 14%.