In this paper, we studied the effects of the orientation of reinforcing fibers of unidirectional CFRP (carbon fiber reinforced plastic) on the natural frequencies and loss-factors of damped vibrations in thin-walled box beams. It is shown that in symmetric reinforcing, longitudinal-buckling-torsional couplings appear in the beam. In case of asymmetric reinforcing, interaction is observed between either longitudinal oscillatory modes or bending ones in two mutually orthogonal planes.