Analysis of natural vibrations of anisotropic box beams is an interesting practical problem, which has never been discussed in detail. Earlier works, as a rule, were restricted to consideration of two–three lower order modes. Such a small number of modes does not allow one to construct the general picture of vibration modes, which is accompanies by numerous mutual transformations. This circumstance has been the reason for writing this work. The first part contains a detailed description of the mathematical model of the problem, and the second part presents computation results and a detailed discussion of them.