Vertical combination of different seismic resisting systems, permitted in the seismic design codes such as ASCE/SEI 7-16, is an effective choice for the structural design of mid- and high-rise buildings. In the seismic analysis of structures, according to the seismic codes, an estimate of the fundamental period of vibration is needed. On other hand, there is no simplified equation to compute the fundamental period of vibration of mixed buildings with vertical combination of different seismic resisting systems. In this study, by using the Rayleigh’s method, a simplified equation for determining the fundamental period of this type of structures is proposed. This equation is verified by the eigenvalue analysis of the mathematical model of the 11 building frames with different seismic characteristics along their height. Furthermore, two different mixed structures composed of steel and reinforced concrete moment resisting frame (MRF) structures as well as of steel (concentrically braced frame) CBF and reinforced concrete MRF structures are examined in this paper. The results indicate that the proposed formula results in the fundamental period of mixed structures with satisfactory accuracy. The largest and average errors in the fundamental period of the buildings under consideration, obtained by the proposed formula, are less than 10% and 6%, respectively, compared to the eigenvalue analysis.