Abstarct: One of the most important assumptions which is being used in analysis and design of buildings against lateral forces is the rigid-floor assumption. Importance of rigidity of floors in appropriate distribution of lateral forces between the lateral load bearing elements and also significant reduction of degrees of freedom of structure are in the analytical calculation. Lateral rigidity of diaphragms depends on several factors such as: type of the structure, dimensions of structure, rigidity and location of lateral load bearing elements, stiffness of frxames, type and thickness of floors, number of stories and etc. so, we should give more and more importance to this assumption. In this study, in order to investigate how filler- joint floors behave, a lot of models in two cases of rigid-floor (master node method) and flexible-floor (finite element method) in linear limitations are analysed and compared. In linear limitation, the rigid-floor assumption has been investigated in short structures and normal structures with the different lateral load bearing systems. In addition to this, the effects of aspect ratio of rectangular plan, the number of bays, and the number of stories on lateral rigidity of diaphragm in normal structures have been studied. Moreover, Iran code principle in determining of rigidity and flexibility of diaphragm which is introduced with ratio of (∆diaphragm/∆story) has been investigated. The results of studies indicate that the amount of this ratio (0.5) is high and it can not be the same for different concrete and steel structures having different lateral load bearing systems, the shape of plan, the number of stories and etc. therefore this ratio should be separated into several cases for different cases and different structures.