Fragility curves are usually used to determine the vulnerability of structures. But defining failure criteria is of great importance in seismic vulnerability which probability of exceeding from these failure criteria is considered as the basis of probabilistic analysis. In the present study, three types of 3D reinforced concrete buildings having 3, 5 and 8 floors with reinforced concrete moment resisting frame structural system having medium formability designed seismically according to Iranian guideline (seismic regulations of Iranian 2800 code and chapter 9 of National Building Regulations, last edition in 2013) with and without the effect of infill considering structural failure (including low-strength concrete) are analyzed using accelerogram (far source) using  OpenSees (Open System for Earthquake Engineering Simulation). Finally, fragility curves of the above-mentioned structures calculated and drawn using Nonlinear Incremental Dynamic Analysis (IDA). The results showed that the infill has an effective role in decreasing lateral stiffness of buildings as much as 20 to 40 percent in proportion to building height