Under seismic loading, floor and roof systems in reinforced concrete (RC) buildings actrnas diaphragms to transfer lateral earthquake loads to the vertical lateral load resistingrnsystem (VLLR). In most cases, horizontal diaphragms are typically assumed to be rigid,rnthus neglecting the effect of their in-plane movement relative to the VLLR system. rnThough number of studies have been conducted on the effect of diaphragm flexibility,rnsome types of buildings are not addressed. For this reason, many researches have to bernconducted to address different types of reinforced concrete buildings. These types ofrnbuildings depend on their aspect ratio, height, shape, shear wall arrangement and so on. rnIn this study, the effect of floor diaphragm flexibility in tall rectangular reinforced concreternbuildings of story G+15, G+25 and G+35 with aspect ratio of 1:3 and 1:4 were studied andrncomparative study of their dynamic response during seismic loading were discussed.rnETABSrnÂ®rn (2017 v 17.0.1) software was used for the modeling and analysis. rnAll buildings were designed according to ES-EN 1992-1-1:2015 and ES-EN 1998-1:2015rnand modeled with rigid floor assumptions and flexible floor diaphragm. Modal responsernspectrum analysis was used to check the buildings response in terms of story shear force,rnmodal periods, story drift and columns shear force. rnIt is concluded from the study that, buildings with aspect ratio 1:4 has larger story drift,rnfundamental period and column shear force received by the columns when modeled withrnflexible floor diaphragm than rigid floor diaphragm assumption. The effect of diaphragmrnflexibility is larger on G+15 story than the G+25 and G+35 story building, but most of all,rnlateral load received by the columns which are found at the middle of the buildings whenrnmodeled with flexible diaphragm shows large increase and these increase may cause clearrnvariation of design output between rigid and flexible diaphragm.