A method for vibration analysis of proportional asymmetric plan frame buildings is presented in this paper. The whole structure is idealized as an equivalent shear-torsion beam in this method. The governing differential equations of equivalent shear-torsion beam are formulated using continuum approach and posed in the form of simple storey transfer matrix. By using the storey transfer matrices and point transfer matrices which consider the inertial forces, system transfer matrix is obtained. Natural frequencies can be calculated by applying the boundary conditions. The structural properties of building may change in the proposed method. A numerical example has been solved at the end of study by a program written in MATLAB to verily the presented method. The results of this example display the agreement between the proposed method and the other valid method given in literature.