During the earthquake, the lateral resistance system of structural building arrives the inelastic region and rather than stiffness eccentricity, which is a suitable criterion for seismic response of structures, the strength eccentricity should be taken into account. In this research, in order to evaluate the stiffness irregularity, 3 steel moment frame buildings in 6, 9 and 12 stories were considered and three regular, stiffness and strength irregular models were simulated. Under the nonlinear time history analysis via the considered earthquakes, the seismic responses were obtained and fragility curves were plotted. Finally, probabilistic interpretation of results has been performed. Survey results indicated that the response of stiffness or strength irregular models have been more critical compared to regular ones. Moreover, the occurrence probability of collapse prevention structural safety level in irregular 6-story models 54 percent and in 9 and 12-story irregular models have been increased as 40 and 19 percent respectively.