The modern reports and research showed that the buildings with more irregularity are less safe in facing earthquakes. this study aims to study the behavior of regular and irregular building in seismic matter. Three irregular shapes (L-Shaped, T-Shaped and C-Shaped) having two types of plan irregularity (accidental torsion and reentrant corner) were compared with rectangular regular one in three seismic hazards (Low, Moderate and High). ETABS v19 is used to model and analyze 12 different models with 15-story and 20-story RC frame structures using dynamic response spectrum method to obtain the maximum displacement, story drift, maximum drift and accidental torsion committing to ASCE7-10 code. The purpose of the study is to alleviate the seismic risk on the irregular structures and make our buildings safer, less cost and more Sustainable. at the end, we provide a table to guide the architectural engineers choosing the best building configuration based on the area seismicity and seismic hazard in order to match the purposes and the structural design in addition to provide some solutions to avoid structural design problems. The results were as follow: In the 15-story RCC frame structures with special moment resisting frame, the L-Shaped buildings are not safe to use in high seismic hazard areas, and the T-Shaped buildings are not safe to use in low and high seismic hazard areas. In the 20-story RCC structures with special moment resisting frame, the L-Shaped buildings are not safe to use in high seismic hazard areas, and the T-Shaped buildings are not safe to use in all seismic hazard areas. The comparison between the irregular shapes and regular one was predicted in L-Shaped and T-Shaped building which have worse seismic behavior than regular shape, but the comparison with C-Shaped was different, the C-Shaped gave a better behavior compared to the rectangular regular building. Keywords: Regular, Irregular, story displacement, L-Shaped, T-Shaped, C-Shaped, story shear, overturning moment, story drift, seismic hazard, ETABS.