The non-linear theory of elastic thin-walled bars of open cross-sections proposed by the auther is applied to the study of large torsion of such bars. The fundamental equations are simplified for the case of bisymmetrical ane central symmetrical cross-sections. For non-symmetrical cross-sections, it is generally impossible to obtain pure torsion without bending in the non-linear theory. The problem is solved by a perturbation method. Two specific examples are considered.