A method of analysis of the effect of two open cracks upon the frequencies of the natural flexural vibrations in a cantilever beam is presented. Two types of cracks are considered: double-sided, occurring in the case of cyclic loadings, and single-sided, which is principle occur as a result of fluctuating loadings. It is also assumed that the cracks occur in the first mode of fracture: i.e., the opening mode. An algorithm and a numerical example are included.