The purpose of this study is to establish the effects of rigid floor assumption on shear forces that occurs on shear walls of building type structural systems. Before the advent of computers only limited number of equations could be solved manually. Thus, simplification methods have been developed to reduce the total processes. In the structural analysis of buildings, rigid floor assumption is one of these methods. At the mass center of each rigid floor, there is a master node having three degrees of freedom to represent the two in-plane translations and one out of plane rotation. In this study, the linear elastic design analysis is carried out modeling floors as rigid diaphragm and flexible diaphragm. Results of the analysis are presented comparatively. According to these results, as a consequence of rigid floor assumption, it can be an issue to get designed such buildings those construction costs are higher but structural safety is lower in comparison with flexible floor model.