Abstract：An approximate method of calculating the fundamental frequency of bending vibrations of multi-cracked beams is proposed on the basis of a modification of the method for single-cracked structures. The deflection of multi-cracked beams is assumed by adding polynomial functions to that of the uncracked beams. This new deflection function satisfies the boundary and the kinematic conditions. The closed-form expressions for the fundamental frequency are obtained by using the Rayleigh method. Its validity is confirmed by comparison with Ansys simulation results.