The contribution of infill walls to lateral stiffness and their effect on the natural vibration period of the buildings has been expressed in many studies. An infill wall’s effect on the natural vibration period of a building is taken into account by empirical formulas in almost all earthquake codes. These formulations are based on the structures’ height and the infill walls are taken into account by a coefficient. However, such parameters like wall thickness, modulus of elasticity of the walls and opening ratio in infill walls are not considered. In this study the effects of the thickness, modulus of elasticity and opening ratio of an infill wall on the natural vibration period are analyzed and the results have been compared with empirical formulas obtained from other studies in the literature.