In this paper, the behaviour of thin-walled sections of a bar with restrained torsion is studied. Neglecting these warping behaviours may generate significant errors, particularly for open profile torsion or shear bending of short beams. The governing equation for non-uniform torsion is used to study the behaviour of the beam with restrained torsion. The variation of the primary torsional moment, secondary torsional moments and warping moments for different value of characteristic number for torsion are presented graphically. Finally, the behaviour and comparison of all torsional moment components with three different thin-walled sections are illustrated by presenting and discussing their results. The section properties, displacements, rotations, stresses and their distribution within the span are compared based on the required value of characteristic number for torsion. It is found that for all thin-walled sections, the characteristic number for torsion is the key criteria for the study of the behaviour of thin-walled sections of a bar with restrained torsion.