In this paper, the problem of vibrations of thin-walled beams is investigated. The fundamental assumption is the same as that in a precceding paper [l], i.e. the non-deformability of contourlines of transverse cross-sections in its own Plane. Equations of vibrations, show that the vibrations of thin-walled beams generally take place as a combination of bending-torsional and longitudinal vibrations.Corresponding to a certain type of bending-torsional vibrations, instead of only one frequency as in the well-known Vlasof's theory, there are an infinite number of frequencies, corresponding to different types of longitudinal vibrations.The natural frequeneies ,as calculated by Vlasof's theory correspond to the lowest frequeneies in our theory, but the former are generally higher than the latter as should be expected.