This article investigated the seismic performance of low-rise steel intermediate moment resistance frame with different percentages of mass irregularity. The irregularity of structures considered according to Iranian standard code No.. 2800 and ASCE7. For this purpose, a 3-story structure that is located on soil type 2 of Iranian standard 2800 (soil type C of ASCE7) with the high-risk level hazard seismic category (region of Tehran), has been studied. The investigated structure had been subjected to nonlinear o time-history analysis under 22 records far-field ground motions considering soil-structure interaction and various mass irregularities at design basis earthquake level of Iranian standard No. 2800. This structure was simulated via OpenSees finite element software that the nonlinear behavior of geometry and materials is considered. The results showed that the drift and absolute acceleration responses of structures with flexible foundations did not have the same pattern compared to structures with rigid foundations. The considered irregularity of structures can be increased the drift value about 30 percent compared to rigid base structure. Also, the maximum absolute acceleration responses can increase about 40 percent with exciting irregularity compared to rigid base structure. In addition, the mean of absolute acceleration responses can increase about 22 percent with exciting irregularity compared to rigid base structure.