This study investigates the flexural-torsional coupled vibration of thin-walled composite beams. We first derived the thin-walled beam theory for an arbitrary cross-section and applied it to a biconvex cross-section. The flexural-torsional coupled motion is generated featuring a symmetric lay-up composite configuration. The natural frequencies are determined by solving the eigenvalue problem, which are obtained both for the warping restraint and the free-warping models. The variations of the natural frequencies with respect to the ply angle orientation are computed for selected values of thickness, slenderness and aspect ratios. We find that the natural frequencies have a tendency to increase with increasing slenderness and thickness ratios, while they tend to decrease with increasing aspect ratio. We also observed that omission of the transverse shear effect, particularly at the ply angles between 60 and 120, led to an overestimation of the natural frequencies of the first and the second modes for higher slenderness ratios, while overlapping results are obtained for any thickness or aspect ratios regardless of including/omission of transverse shear deformation. Overall, we point out that the geometrical aspects have a substantial influence on dynamic characteristics in the design of thin-walled composite beams. (C) 2015 Elsevier Ltd. All rights reserved.