Most recent seismic codes include response modification factors (R in US practice) orrnbehavior factors (q in European practice) in the definition of equivalent lateral force methodrnof analysis used for the design of earthquake resistant buildings. Seismic design makes usernof energy absorption and dissipation to reduce the design forces in order to achieve economy.rnThe behavior factor (q) or response modification factors (R) plays a central role in seismicrndesign process, since it is used to reduce the linear elastic design spectrum to account for thernenergy dissipation capacity of the structure. In this study the evaluation of behavior factor forrnregular and irregular ductile reinforced concrete structures using nonlinear analysis isrninvestigated. The evaluation is done by designing regular and irregular structures for differentrnductility classes with the provision of our seismic code and by checking their performancernthrough non-linear analysis using CSI ETABS. In this paper parametric studies have beenrnconducted and the effects of different parameters (seismic location, ground type, number ofrnstories, ductility class and 3D modelling effects) on behavior factor are presented in detail.rnMoreover, the evaluation of behavior factor using different models under favorable andrnunfavorable conditions along with performance comparison of structures designed forrndifferent ductility classes were done and the results are discussed and presented in this study.rnFurthermore, the evaluation of behavior factor for 3D plan irregular reinforced concreternstructure under unfavorable conditions is done using non-linear dynamic analysis and thernfindings are presented as well.