Abstract: In this paper, considering that stresses along wall thickness are distributed non-uniformly, a theory of torsion analysis of composite thin-wall boxbeams without external restraint, based on the theory of composite laminated plates, was developed. In the theory, various coupled effects were taken into account and formulas of deformation and stress were presented. The results of analysis indicate that due to coupled effects, in general, the pure torsion of composite boxbeam may not exist, so a concept of torsion without external restraint is suggested. Finally, the numerical results are verified and analysed.