Frame system structures which composed of only reinforced concrete columns, beams and slabs, have been widely adopted for many framed buildings. Generally, in-plane stiffness of slabs is ignored in the conventional analysis of such structures. However, in reality, the floor slabs may have some influence on the lateral response of the structures. Consequently, if the in-plane stiffness of slabs in a frame system structure is totally ignored, the lateral stiffness of the global frames may be underestimated. Therefore, the objective of the research is to investigate the effect of floor diaphragms in multi-story frames by comparing frames models with different slabs thickness by those without slabs. Furthermore, it can be seen from the study that the slab thickness is an important factor increasing in-plane stiffness of the slab and consequently increasing the overal in-plan stiffness of the building leading to an increase in base shear and a decrease in lateral displacements values.