At the present lime most current building codes specify equations to be used for the calculation of the base shear and lateral loads. For the determination of these lateral loads, it is necessary to determine first the period of vibration of the building. As is known, the period of vibration of buildings can be determined theoretically or experimentally. The theoretical design values of the period ol' vibration are different from the experimental ones. Significant differences have been observed for RC multistory frame buildings and in a lesser degree for other building types such as: masonry, stone, large-paneled, and large-brains buildings. The natural periods of vibration of 53 multistory RC frame buildings have been measured by the station of Engineering-Seismometer Services (ESS). These 53 buildings have different number of stories, different dimensions in.plan (with different plan length to width ratios) and effected on different soil conditions. On the basis of these data an improved simple empirical equation relating the period of vibration and the number of stories of the building is proposed 113 estimate the fundamental period of vibration of RC multistory frame buildings. In this paper the effect of soil condition and the divisions of the building in plan on the period of vibration have been studied. Comparison of periods determined using the proposed relationships and the adopted ones shows rather well agreement.